[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.

The current question is if there should be different build scripts for all of the different CLR Languages. They are all built identically but Visual Basic and F# both require their own Debian package. A single CLR solution file can contain, C#, F#, and VB.

### 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. 
@Joshua-Anderson Well, you can always install F#/VB by doing "apt-get install" in the `script` even if you set `language: csharp`.

I honestly don't know how common it is to mix languages in a project, I'd estimate it's only a tiny fraction and even those will likely use one "main" language, e.g. C#, so setting `language: csharp` and installing F# in the `script` sounds reasonable I think.

The strong argument in my opinion for splitting it into `csharp`, `fsharp` and `vb` is that it's consistent with all the other languages supported by Travis. I mentioned `scala`/`java` before, but a better example might be `php`, Travis [supports](http://docs.travis-ci.com/user/languages/php/#Choosing-PHP-versions-to-test-against) Facebooks' HHVM by doing `language: php` and setting `php: hhvm`. So HHVM is essentially what Mono is to .NET.