[WIP] Added basic C# support

This adds basic C#/mono support for traivs-ci

Syntax
``` yml
language: csharp
mono:
  - mono3
  - mono2
  - mono3.6.0
solution: helloworld.sln
```

This is very WIP.

At the moment there is no linux support, this works on Mac OS X workers only.

I can't find a good way to install mono on the linux workers.

The two options I can think of are:
 
- Debain Package: No easy way to install a specific version of mono
-Compiled tarball: We could compile and host our own version of mono. I don't like this solution is it adds the strain of building various versions on mono (but isn't very difficult to do), but because mono's binaries are dependent on location. So if you build the project to be used at `/home/travis/travis-ci/travis-mono/` the project would break if it was installed at `/home/travis/mono/`.

So guys, what is the lesser of two evils on the linux side? Any other ideas on how to install mono?

### F# and Visual Basic

The Mac Mono package includes F# and Visual Basic runtimes. How do we plan to implement these runtimes on linux?

Is there any other work or PR's that needs to be done for Travis to properly interpret the `csharp` language?