Make 'await' resolve as in the C# 5.0 language specification.

This means that the awaiter type must implement INotifyCompletion and can optionally implement ICriticalNotifyCompletion.
Fixed await with generic awaiter types.
Test demonstrating that a const field of type decimal is not considered constant by the cecil loader.
CecilLoader: Import fields with a DecimalConstantAttribute as 'const'
Copy over some fixes from SD5.
Fix InvalidCastException in InconsistentNamingIssue.
Merge pull request #128 from erik-kallen/await-csharp5-final

Make 'await' resolve as in the C# 5.0 language specification.
Merge pull request #132 from erik-kallen/decimal-constant-bug

Decimal constant bug
Add "add using" context action.

The unit tests are based on pull request #104 by Adam Connelly
Fixed try catch statement position + failing test.
Update NRefactory to v5.2.0-554-g755e37e
Enable "Add using" context action.