Fix NRE exception on syntax coloring

Close #564.

I'm able to reproduce the stacktrace on VS unit tests using this code fragment:

![image](https://cloud.githubusercontent.com/assets/941060/3726659/961335a8-1695-11e4-932a-ef05914e88c4.png)

Here is the simulated stacktrace:

> Test Name:	should be able to get classification spans
Test FullName:	FSharpVSPowerTools.Tests.SyntaxConstructClassifierTests.should be able to get classification spans
Test Source:	C:\PowerTools\tests\FSharpVSPowerTools.Tests\SyntaxConstructClassifierTests.fs : line 58
Test Outcome:	Failed
Test Duration:	0:00:00.429
Result Message:	System.NullReferenceException : Object reference not set to an instance of an object.
Result StackTrace:	
at <StartupCode$FSharpVSPowerTools-Logic>.$SyntaxConstructClassifier.clo@43-47.Invoke(ProjectItem _arg1) in C:\PowerTools\src\FSharpVSPowerTools.Logic\SyntaxConstructClassifier.fs:line 44
at <StartupCode$FSharpVSPowerTools-Logic>.$SyntaxConstructClassifier.clo@41-44.Invoke(Unit unitVar) in C:\PowerTools\src\FSharpVSPowerTools.Logic\SyntaxConstructClassifier.fs:line 42
at FSharpVSPowerTools.MaybeBuilder.Delay[T](FSharpFunc`2 f) in C:\PowerTools\src\FSharpVSPowerTools.Core\Utils.fs:line 160
at FSharpVSPowerTools.SyntaxColoring.SyntaxConstructClassifier.getProject() in C:\PowerTools\src\FSharpVSPowerTools.Logic\SyntaxConstructClassifier.fs:line 40
at FSharpVSPowerTools.SyntaxColoring.SyntaxConstructClassifier.updateSyntaxConstructClassifiers(Boolean force) in C:\PowerTools\src\FSharpVSPowerTools.Logic\SyntaxConstructClassifier.fs:line 62
at <StartupCode$FSharpVSPowerTools-Logic>.$SyntaxConstructClassifier.-ctor@131-67.Invoke(Unit unitVar0) in C:\PowerTools\src\FSharpVSPowerTools.Logic\SyntaxConstructClassifier.fs:line 131
at FSharpVSPowerTools.ProjectSystem.VSUtils.DocumentEventListener..ctor(FSharpList`1 events, UInt16 delayMillis, FSharpFunc`2 update) in C:\PowerTools\src\FSharpVSPowerTools.Logic\VSUtils.fs:line 265
at FSharpVSPowerTools.SyntaxColoring.SyntaxConstructClassifier..ctor(ITextDocument doc, IClassificationTypeRegistryService classificationRegistry, VSLanguageService vsLanguageService, IServiceProvider serviceProvider, ProjectFactory projectFactory, Boolean includeUnusedDeclarations) in C:\PowerTools\src\FSharpVSPowerTools.Logic\SyntaxConstructClassifier.fs:line 130
at FSharpVSPowerTools.SyntaxConstructClassifierProvider.<>c__DisplayClass1.<GetClassifier>b__0() in C:\PowerTools\src\FSharpVSPowerTools\SyntaxConstructClassifierProvider.cs:line 390
at Microsoft.VisualStudio.Utilities.PropertyCollection.GetOrCreateSingletonProperty[T](Object key, Func`1 creator)
at FSharpVSPowerTools.SyntaxConstructClassifierProvider.GetClassifier(ITextBuffer buffer) in C:\PowerTools\src\FSharpVSPowerTools\SyntaxConstructClassifierProvider.cs:line 389
at FSharpVSPowerTools.Tests.SyntaxConstructClassifierHelper.GetClassifier(ITextBuffer buffer) in C:\PowerTools\tests\FSharpVSPowerTools.Tests\SyntaxConstructClassifierTests.fs:line 24
at FSharpVSPowerTools.Tests.SyntaxConstructClassifierTests.should be able to get classification spans() in C:\PowerTools\tests\FSharpVSPowerTools.Tests\SyntaxConstructClassifierTests.fs:line 62