Adding k-mer and q-mer counting.
Merge pull request #269 from fnothaft/count-kmers-qmers

Adding k-mer and q-mer counting.
Created command categories
Merge pull request #272 from karenfeng/command-categories

Created command categories
Fixes issue in reference models where strings are not sanitized on collection from avro.
Adding fixed depth prefix trie implementation, as part of read error correction work.