Fix a couple typos in py2 py3 doc
Merge pull request #796 from anderspitman/py2_3_typos

Fix a couple typos in py2 py3 doc
Merge branch 'master' into move_find_features
Remove find_features from BiologicalSequence

This functionality was previously moved into the subclass
NucleotideSequence.
Remove find_features test for BiologicalSequence

This functionality was removed from the class.
Remove feature_types from NucleotideSequence

It was only used in find_features, so we localized the
functionality there.
Use gap_alphabet in find_features

Replaces a local implementation that didn't handle '.' gaps.
Add tests for allow_gaps in find_features
Specify available feature types in find_features