Initial version of Visual Studio unit tests

Open the pull request for dicussion.

I added the first test using `TestUtilities` project at https://pytools.codeplex.com/SourceControl/latest#Common/Tests/Utilities/Mocks/MockTextBuffer.cs.

I'm able to construct `SyntaxConstructClassifier` instance for testing, but it requires a lot of stub generation. I hit a problem where I can't use `MockTextBuffer` from `TestUtilities` due to the error message:

> Test method FSharpVSPowerTools.Tests.ClassifierTests+ClassifierTests.should be able to get classifier spans threw exception: 
System.InvalidCastException: Unable to cast object of type 'TestUtilities.Mocks.MockTextBuffer' to type 'Microsoft.VisualStudio.Text.ITextBuffer'.

I had to resort to create a separate `MockTextBuffer` type in `FSharpVSPowerTools.Tests` project, which requires a lot of effort to match the same functionalities of the C# version. The error message is silly since TestUtilities.Mocks.MockTextBuffer implements Microsoft.VisualStudio.Text.ITextBuffer interface. Could it be a problem because I use this C# mock class in F#?

Can someone take a look?
I don't like the fact that a mutable collection is exposed publicly. What about adding `Register: string * obj -> unit` method?