adding syn_selection_by_codon module

[fwelsh]

Added a new module which identifies sites with selection on synonymous codons, using Fisher's Exact test to calculate codon enrichment at each site.

[jbloom]

@fwelsh: Can you make and then push a few additional changes before I merge this:

1. Change function name to something syn_selection_by_codon in the new syn_synonymous module. The reason is that we are currently doing a different test for each module; in a future version we might test by amino acid (using 2 X N table), and that would by syn_selection_by_aminoacid.

2. Add spaces after the initial - here, otherwise the docs won't render correctly as a space is required after list items.

3. Update the CHANGELOG.rst file to indicate that in the new version 2.6.2 you have added the syn_selection module.

4. Update the version number here to 2.6.2.