[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. 
Just a heads-up: this will work for now because 3.8.0 is the current version in the repo, but when a new version is pushed the old version is currently removed from the repo. I've asked @directhex to retain the old version and he said he's looking into that as well.