First working SpriteFontDescription .xnb written

Note: This is https://github.com/mono/MonoGame/pull/1322 resubmitted into the correct branch.

This pull request includes a couple of things.

First, SpriteFontDescription is working! 

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 and List 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.