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.
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