Implement rename refactoring

Close #20.

This is a rough version which is able to rename a single file via a Rename Dialog.

Some questions are:

 - How can we open documents in a project inside VS programmatically in order that after renaming, we could take advantage of undo mechanisms in VS?
 - How can we get hold of built-in Refactor -> Rename menu? It's disabled for F# by default. We currently add an ad hoc context menu item which has no icon and key bindings.
 - What should be UI workflow for inline refactoring? If scope of the symbol is the current file, we activate inline editing via Rename menu item. When do we stop inline editing?