Added F# SymbolHelper.fs. Fixed some of the highlighting.

This resolves the following issues:
#698 - Union types not highlighted correctly in semantic highlighting
#697 - Active patterns not highlighted in semantic highlighting
#696 - #endif is not highlighted correctly in semantic highlighting

Known Issues: Computation Expressions do not highlight correctly as FCS
doesn't report them as Computation Expressions.
Merge pull request #700 from ashtonkj/master

Added F# SymbolHelper.fs. Fixed some of the highlighting.