Consolidate CSharp and VB.NET gitignore files into VisualStudio one.

__Don't merge until we've had some discussion first__  :hamster:

The `CSharp.gitignore` and `VB.NET.gitignore` files are pretty much exactly the same. Most of the settings in there mimic the existing global `VisualStudio.gitignore`. I saw another PR to add an `FSharp.gitignore` that's almost exactly the same as these others.

Rather than have an ignore file for every .NET language, I think we should just have one for `VisualStudio`.

For a moment, I thought it should be called `DotNet.gitignore` but if you're doing C++ development in VisualStudio, that wouldn't apply. Also, for all intent and purposes, Visual Studio is the de-facto IDE for developers doing .NET, C++, or Windows RT development on Windows. Other development tools tend to adopt its conventions. If we learn that MonoDevelop produces build files we want to ignore that aren't included in this file, we can consider adding them here.

Remember, when you create a new repository on github.com, the list of gitignore files you can use is populated from this repository.

![gitignore-files](https://f.cloud.github.com/assets/19977/23376/c4fb6a0a-4a38-11e2-9208-0bc9562d2290.png)

__Questions__

* What do you search for when using that dropdown?
* Does `VisualStudio` seem like the right name?
* If not, do you have a better name?