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.
Is there an underflow / floating-point issue here?  Is there a reason you're not just doing this computation in log-space? (i.e. omit the toSuccessProbability call above, and then q.reduce(_ + _) here instead?)