[MonoDevelop] Mercurial support

[Levi Bard]

>  * The .addin.xml doesn't seem to get copied to the build dir, which means
> you can't just bundle it with "mdtool setup pack
> MonoDevelop.VersionControl.Mercurial.addin.xml -d:" in bin/Debug/

The addin.xml gets embedded into the assembly.

>  * The .mpack file doesn't contain the Mercurial.dll, so the context menu
> causes an error

I'm not using the Makefile yet, just building from MD, so I'm not too
surprised that it's not working.