Issue877 vim 73 support

Issue #877 

Manual testing performed (in both 7.3 and 7.4):
- Add F# errors, check highlighted on write
- Call FsiEval with trivial request
- Call RunProject
I'm a bit stuck. Removing the CursorHold handler exposes an issue: I have to save twice to make the errors appear (or disappear). 

I think what is happening is this: if the buffer is large, when SyntasticCheck is called on the first BufWritePost event, parsing hasn't been completed. By the second BufWritePost event, parsing has completed and errors are found.

Keeping the CursorHold handler in was actually suppressing this issue by running SyntasticCheck again. 

I'm not sure how best to resolve this - obviously don't want to block until parsing is finished, but don't want to be polling on every CursorHold event either. Could have BufWritePre set a flag like g:fsharp_parse_started and have OnCursorHold run SyntasticCheck if this flag is set, but that's a bit ugly. Any advice?