mostly address the additions for 0.1.0

~~**NOT READY FOR MERGE**~~

This addresses a few issues:
- addition of ``ProteinSequence`` object (#25)
- moved some methods from ``NucleotideSequence`` to ``BiologicalSequence`` that were also useful for ``ProteinSequence``
 - documentation to illustrating translation of a ``NucleotideSequence`` to a ``ProteinSequence`` (#25); this was done in favor of addition a ``translate`` method because this is already present in the ``GeneticCode`` objects, and after some discussion with @ElBrogrammer we decided that it doesn't necessary make sense for a ``NucleotideSequence`` to have a ``GeneticCode`` but rather to use a ``GeneticCode``. There will be more work on this for 0.2.0, and discussion of that should happen in #337. 
- ``GeneticCode.translate`` and ``GeneticCode.sixframe`` now return a ``ProteinSequence`` or list of ``ProteinSequence``s, respectively (#326)
- ``identifier/identifiers`` replaced with ``id/ids`` in core objects (#25)
- some minor updates to the ``GeneticCode`` documentation, one method name, and some variable names, but more to follow for 0.2.0 (see #337)