Adding VS2015 support for ServiceHost project

Creating the standard sln flies for the root of the sqltoolsservice
project and adding xproj files for the individual projects.

Has been confirmed to open and build with VS2015. xUnit tests will work,
but only from Test Explorer (ie, ReSharper test runner doesn't work with
dnx)

Note: This may be subject to change as new standards for .NET Core project
files are developed. See dotnet/roslyn-project-system#37 for more details.

global.json

@@ -1,5 +1,8 @@
 {
-  "projects": [ "src", "test" ]
+  "projects": [ "src", "test" ],
+  "sdk": {
+    "version": "1.0.0-preview2-003121"
+  }
 }