[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.

I can't use the offical debian repo for mono, as Ubuntu 12.04 is too old, so I'm using a community repo to install the latest version of mono on Ubuntu 12.04. This prevents us from installing visual basic and F#. We can switch to the official repo when we update to Ubuntu 14.04. Then we can install the Visual Basic and F# runtimes on linux.

### 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. 
Thanks for the tip. I don't think that locking the version is too important, but I figured it would be a good idea for the Mac workers and the Linux workers to use the same version of mono. I'll remove it for now and if you do decide to save old versions of mono I'll add it back in.