Use TextBuffer in VirtualProjectProvider so that it works for dirty files

Fix #452.
Merge pull request #468 from dungpa/overhaul-language-service

Use TextBuffer in VirtualProjectProvider so that it works for dirty file...
Merge remote-tracking branch 'fsprojects/master' into graying-unused-declarations

Conflicts:
	src/FSharpVSPowerTools.Logic/SyntaxConstructClassifier.fs
	src/FSharpVSPowerTools.Logic/VSLanguageService.fs