[WIP] Added basic C# support

This adds basic C#/mono support for traivs-ci

Syntax
``` yml
language: csharp
csharp:
  - mono
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. 
What do you want to achieve with this property? What's the difference to TRAVIS_MONO_VERSION?