Added Pixel, SimpleCodec classes; renamed encoders module

* Replaced integer triples with Pixel objects
* Drew simple_encode, simple_decode functions into SimpleCodec class
* Renamed transmedia.encoders as transmedia.codecs
* Updated tests
* Minor change in docs makefile
* Bumped version to 0.1.7
Merge pull request #12 from jonnystorm/add_pixel_object

Added Pixel, SimpleCodec classes; renamed encoders module