Adding k-mer and q-mer counting.

This adds functions for _k_-mer and _q_-mer counting, which is necessary for error correction. Additionally, this adds commands that support counting _k_/_q_-mers from the CLI.

_q_-mers are quality score weighted _k_-mers, and are described in http://genomebiology.com/2010/11/11/R116/abstract.
There isn't an underflow issue when you're looking at short _k_-mers. If you have a 100-mer with all bases at Q20, your _q_ weight for the _q_-mer is about 0.35.

Also, underflow isn't such an issue for _q_-mer counting. The goal is to get an expectation for the count of a _k_-mer, therefore _q_ weights that underflow wouldn't have contributed much to the expectation anyways.