Added Huffman encoder/decoder + alpha blending

I've implemented a lossless encoder/decoder for images, using Huffman compression. This may offer you greater freedom in what GUI elements you want to include. Huffman compression offers good space/time properties: GUI images tend to compress well as they consist mostly of a few colors, and the decoder only requires ~ 1 table lookup + a bunch of shifts/ands per pixel.

I've also implemented 5-bit alpha blending, which can be combined with the Huffman decoder to display GUI elements with curved, anti-aliased edges:
![example](https://cloud.githubusercontent.com/assets/1388817/4408969/c9b6b9c2-44d8-11e4-8277-e53709d74339.png)

These commits have been tested successfully

 in a local test harness, but not on actual arducorder mini hardware.