Unittest for Bio.GenBank.read cases

I added additional tests for Bio.GenBank.read() into the
``test_GenBank_unittest`` file as I couldn't figure out how to implement them
in the old-style test file.
Test coverage for Bio.GenBank went from 78% to 81%.
Finally adopt string based equality testing for sequences.

This also solves Python 2 specific usage of __cmp__ meaning that
for Python 3 the objects defaulted to purely using id comparison.

See also https://redmine.open-bio.org/issues/2351
Document sequence comparison changes
Mention Bio.Seq sequence equality changes in DEPRECATED