[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. 
@nterry I didn't want to use `csharp_runtime` because this runtime is shared between all CLR languages. It didn't make much sense to me to have VB and F# projects setting `csharp_runtime`. What do you think?