Merge pull request #103 from BinPro/develop

Version 0.3.3
More tests for composition calculation + refactoring for ease of testing
Use Counter for faster kmer counting
Do kmer in feature_mapping check later, and fewer times
Use kmer tuple instead of string for speedup
Do length filtering early, refactoring input
Speedup using numpy for counting kmers
Merge pull request #105 from alneberg/optimize_input

self.merge()