fontified eldoc - requires csharp-mode

This change uses a dedicated buffer running csharp-mode to fontify eldoc strings from omnisharp which I have found to be a nice feature.

The kicker about this is that it requires csharp-mode which until now, omnisharp was not dependent on. Alternatively, if you want omnisharp to be major-mode agnostic, there could be a custom variable allowing the user to specify what major mode they use omnisharp with and use that for the fontification buffer.

I have found this feature helpful for me, let me know if you think it would be a good fit for the package.

![colored-eldoc](https://cloud.githubusercontent.com/assets/1441728/2667802/506425fa-c0b2-11e3-8f63-7028f8b5ed54.gif)