Pathed document initial cut

Initial cut of pathed document.  Please review, break, make suggestions etc.  I don't want my code to go stale so the best bet is to get this here for visibility.

*   I think there might be a bug with multiple open documents getting the wrong path
*   F# specific Icons for the various types to make them distinct, like module for instance.
*   I think storing the AST within the parsed document could pave the way forward for getting a smooth refactoring implementation.