rework vim plugin to integrate into syntastic

Hi,

I just did a quick update of the vim plugin files to reuse the existing *on-the-fly* code/type checking functionality of the [syntastic](https://github.com/scrooloose/syntastic) vim plugin. To my mind there is no need to reinvent all of the error highlighting/code checking functionality just for this plugin. As soon as you have *syntastic* in vim's runtime path code checking for F# should work out of the box.

I will add some documentation to the README soon.

Cheers,
Gregor
just tested that and it works nicely.