[WIP] Added basic C# support

This adds basic C#/mono support for traivs-ci

Syntax
``` yml
language: csharp
solution: helloworld.sln
```
### F# and Visual Basic

The Mac Mono package includes F# and Visual Basic runtimes.

The current question is if there should be different build scripts for all of the different CLR Languages. They are all built identically but Visual Basic and F# both require their own Debian package. A single CLR solution file can contain, C#, F#, and VB.

### Docker Images

This won't work with the docker containers because I can't find way to install mono on Ubuntu without a Debian package. This could be resolved be preinstalling the mono Debian package. If anyone knows of any mono tarball binaries or anything like that let me know. 
As far as I see it, this is now just used to store the xamarin.gpg file, wouldn't it be a bit cleaner to just store it in /tmp and not create a folder here?