Add pixel conversion code

[JayFoxRox]

There should be routines which convert NV2A pixel data to a more usable format. Possibly PIL images or A8R8G8B8 bytes(). I prefer the PIL solution because almost everyone will have it anyway, and it supports DXT.
PIL could be an optional dependency.

There's work in nv2a-trace which will eventually move here. See Texture.py in nv2a-trace (current version linked, please look where file has moved in master branch)
Please work on nv2a-trace if you want to solve this issue.

There is also some dirty unswizzling code in nv2a.py which must be looked at, see #6