Rename codec and codec_errors to encoding and encoding_errors respectively.
Disable Python 3.2 builds in Travis as changes only support 2.6, 2.7 and 3.3+.
Don't define __bytes__(), and don't define __unicode__() on Python 3.

The previous attempt to add unicode support tried to make Python 2 and
3 behave in the same way apart from __str__().  This commit makes the
modules, and the tests, provide only __str__() on Python 3 and both
__str__() and __unicode__() on Python 2.