Create MS.SqlTools.Credentials project (#249)

* Initial credential service files

* Clean-up hostloader

* Update build scripts to build credentials archive

* Move hosting files to new assembly

* Add credentials files to MS.SqlTools.Credentials

* Remove duplicate files

* Update namespace in program.cs

* Fix test build breaks

* Update extensions visibility.

* Remove unused resource strings

* Add xproj files to SLN for appveyor builds

* Fix appveyor build break in test project

* Fix extensibility tests

* Fix various typos in latest iteration

* Add settings for Integration build

* Fix codecoverage.bat to use full pdb for new projects

* Fix bug when packing in folder with native images

* Fix typos in xproj

* Reset XLF to fix build.cmd

test/Microsoft.SqlTools.ServiceLayer.TestDriver/project.json

@@ -9,6 +9,12 @@
     "dotnet-test-xunit": "1.0.0-rc2-192208-24",
     "Microsoft.SqlTools.ServiceLayer": {
       "target": "project"
+    },
+    "Microsoft.SqlTools.Hosting": {
+      "target": "project"
+    },
+    "Microsoft.SqlTools.Credentials": {
+      "target": "project"
     }
   },
   "testRunner": "xunit",