Roslyn completions

Restore the augmented code completion support to Visual Studio 2013 with Roslyn installed.

* Add the `ToolTipProvider` class to handle tool tips for non-Roslyn completion items (fixes the exception which shows an error dialog prior to the **2014.5.31 Beta** release.
* Add an implementation of `IOleCommandTarget` to bypass the Roslyn keyboard handling and manually trigger the correct completions when tab, return, `/`, or `>` is typed and a non-Roslyn completion item is currently selected.