QuickLook: Handle thumbnail generation

This is a very quick and dirty implementation to address #1226.

Read 2Kb max of a file and render the attributed string in a bitmap context.
- The background is forced to white, could take the background of the TextMate but it's a bit ugly when rendered.
- I arbitrarily chose Courier size 10 to render the text, and not the font for the TextMate (because I use a 20pt font, and then we don't see much)

Could probably be way better, there's a lot of code dupe.