Feature/dither

Added a variety of black/white dithering algorithms, to give that late 80s/early 90s 1 bit look.  With a little more work, this can be extended to use a limited palette of colors instead of just black/white, though it will be a little more complicated for the user to configure.

Still something of a novice with CoffeeScript, so please let me know if there are any changes you would recommend.
![screen shot 2014-12-23 at 12 11 29 pm](https://cloud.githubusercontent.com/assets/1160174/5542578/e471e3cc-8a9c-11e4-81a7-c13cfb10aad0.png)