Add recipe for the omnisharp package

This is a recipe for [omnisharp-emacs](https://github.com/sp3ctum/omnisharp-emacs/).

It provides Emacs bindings for [OmniSharpServer](https://github.com/nosami/OmniSharpServer) - a
server providing IDE-like features to any text editors for csharp
files and projects. This gives users the benefits of clever code
completion, navigation and more - all with the editing power of Emacs.

I am the maintainer of the project, and would love to see it as a part
of MELPA. The current way to install the package is too difficult,
while installing from MELPA is easy.

I've never submitted a recipe before, but I tried making sure it
builds and installs properly. I used the instructions at
https://github.com/milkypostman/melpa#testing as a basis for the
testing.

Please let me know if there is something that needs fixing! :)