Overhauled FSharpInteractive to take into account various edge conditions

Removed debug diagnostics as commands are already logged in MonoDevelop.
Inherited from abstract base class to save implementing useless update
command.
Altered InteractiveCommand so that if FSharpInteractive is not available
then the commands are visible but disabled.  This can happen because of
various edge conditions if the addin is reinstalled and its not possible to recover.