[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.

That will lead to confusion for any fsharp users... Split them up

> 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.

I think thats true. But still, splitting them up would be more future proof if F# takes off.

> 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 Facebooks' HHVM by doing language: php and setting php: hhvm. So HHVM is essentially what Mono is to .NET.

Thats an idea, dotnet as the language and then doing lang: c# or f# or vb... But i'm still liking the idea of splitting them up


> Well I think it would make the intent more clear, as I said, the languages are called C#, F# and VB, not DotNet.
> Another reason for splitting it up: currently there is no fsharp Debian package from Xamarin. If we go down the language: dotnet route then F# would work on Mac but not on Linux, which is confusing. By splitting it into csharp and vb we can leave out fsharp until there is a package and it'd be clear what is "supported" and much less confusing IMHO.

Ultimately, i propose splitting them up with a view to the future of integrating them when Xamarin provides an F# package. make a csharp, fsharp, and vb class and make them all inherit from the dotnet class