First working SpriteFontDescription .xnb written

This pull request includes a couple of things.

First, SpriteFontDescription is working! 
![FontComparison](https://f.cloud.github.com/assets/1228707/140685/dd91274e-7229-11e2-9f2d-f29ec0d43888.png)

At the moment it only outputs the texture in an uncompressed format. I left it this way for now because I was unsure of how we'd select the compression  for the font. Will it be via a property similar to how the Texture2DProcessor works? Either way, since Compression is already implemented, this is a rather quick fix.

At the moment, using generics in the pipeline was not working. Long story short, we are doing a dictionary lookup for readers and writers, but at runtime we can't use this lookup against an instance of this generic type. As far as the dictionary lookup is concerned, List<T> and List<Rectangle> aren't the same thing. I came up with a solution, but am unsure if it's the best one.

There are still some improvements to implement, but this is a good starting point.