Credentials store API (#38)

* CredentialService initial impl with Win32 support

- Basic CredentialService APIs for Save, Read, Delete
- E2E unit tests for Credential Service
- Win32 implementation with unit tests

* Save Password support on Mac v1

- Basic keychain support on Mac using Interop with the KeyChain APIs
- All but 1 unit test passing. This will pass once API is changed, but checking this in with the existing API so that if we decide to alter behavior, we have a reference point.

* Remove Username from Credentials API

- Removed Username option from Credentials as this caused conflicting behavior on Mac vs Windows

* Cleanup Using Statements and add Copyright

* Linux CredentialStore Prototype

* Linux credential store support

- Full support for Linux credential store with tests

* Plumbed CredentialService into Program init

* Addressing Pull Request comments

test/Microsoft.SqlTools.ServiceLayer.Test/Credentials/Win32/CredentialSetTests.cs

@@ -0,0 +1,99 @@
+//
+// Code originally from http://credentialmanagement.codeplex.com/, 
+// Licensed under the Apache License 2.0 
+//
+
+using System;
+using Microsoft.SqlTools.ServiceLayer.Credentials.Win32;
+using Microsoft.SqlTools.ServiceLayer.Test.Utility;
+using Xunit;
+
+namespace Microsoft.SqlTools.ServiceLayer.Test.Credentials
+{
+    public class CredentialSetTests
+    {
+        [Fact]
+        public void CredentialSetCreate()
+        {
+            TestUtils.RunIfWindows(() => 
+            {
+                Assert.NotNull(new CredentialSet());
+            });
+        }
+
+        [Fact]
+        public void CredentialSetCreateWithTarget()
+        {
+            TestUtils.RunIfWindows(() => 
+            {
+                Assert.NotNull(new CredentialSet("target"));
+            });
+        }
+
+        [Fact]
+        public void CredentialSetShouldBeIDisposable()
+        {
+            TestUtils.RunIfWindows(() => 
+            {
+                Assert.True(new CredentialSet() is IDisposable, "CredentialSet needs to implement IDisposable Interface.");
+            });
+        }
+
+        [Fact]
+        public void CredentialSetLoad()
+        {
+            TestUtils.RunIfWindows(() => 
+            {
+                Win32Credential credential = new Win32Credential
+                                            {
+                                                Username = "username",
+                                                Password = "password",
+                                                Target = "target",
+                                                Type = CredentialType.Generic
+                                            };
+                credential.Save();
+
+                CredentialSet set = new CredentialSet();
+                set.Load();
+                Assert.NotNull(set);
+                Assert.NotEmpty(set);
+
+                credential.Delete();
+
+                set.Dispose();
+            });
+        }
+
+        [Fact]
+        public void CredentialSetLoadShouldReturnSelf()
+        {
+            TestUtils.RunIfWindows(() => 
+            {
+                CredentialSet set = new CredentialSet();
+                Assert.IsType<CredentialSet>(set.Load());
+
+                set.Dispose();
+            });
+        }
+
+        [Fact]
+        public void CredentialSetLoadWithTargetFilter()
+        {
+            TestUtils.RunIfWindows(() => 
+            {
+                Win32Credential credential = new Win32Credential
+                                            {
+                                                Username = "filteruser",
+                                                Password = "filterpassword",
+                                                Target = "filtertarget"
+                                            };
+                credential.Save();
+
+                CredentialSet set = new CredentialSet("filtertarget");
+                Assert.Equal(1, set.Load().Count);
+                set.Dispose();
+            });
+        }
+
+    }
+}