Add quality attribute to BiologicalSequence.

Defaults to None. If present, is stored as a 1-D ndarray of integers.

Still need to add tests, update __eq__, __str__, etc. and make sure that quality is correctly propagated in methods that return new BiologicalSequence objects.
Update BiologicalSequence.__eq__ to include quality scores
Respect quality scores in degap and __getitem__

Also added BiologicalSequence.has_quality().
upper/lower propagate quality scores
Propagate quality scores in BiologicalSequence.nondegenerates()
Propagate quality scores in complement/reverse complement
Add BiologicalSequence.equals method and sequence property
Rewrite __eq__ to use equals method
Add tests for equals method
Fix pep8/flake8 errors
Add tests for sequence property
Add tests for quality and has_quality
equals now forces check of underlying sequence of characters
update lower() unit tests to test ids, description, and quality scores
update upper() tests to account for id, description, and quality scores
Update complement() tests to use equals method and test quality scores
update reverse_complement() tests to use equals method and test quality scores
update is_reverse_complement() tests to check quality scores
Add support for empty and scalar quality scores

Also add tests for degap().
Add tests for __getitem__ with quality scores