Ignore derived on Python2.
Add custom 'binary' type for use in Python2 apps.

Derived from 'bytes', the 'binary' type allows Python2 apps to pickle
binary data using opcodes which will cause it to be unpickled as 'bytes'
on Py3k.
Build the Python 2.7 extension on Python2.
Revert Python2-only tests in the pickletester forked from Py3k .
Under Py3k, 'binary' is an alias for 'bytes'.

Also, update changelog to show changes merged from the 'py2_explicit_bytes'
branch.
Move Py3k pickle/pickletools and related tests into '_3' modules.

Leave behind modules which selectively import based on sys.version_info.
Bring in the Py2-specific Python code from the 'py2_explicit_bytes' branch.

The top-level 'zodbpickle.pickle' module exports the standard 'pickle'
module API for all supported versions of Python (2.6, 2.7, 3.2, and 3.3).
This includes 'protocol 3' support on Python 2.x.
Document changes made folding in 'py2_explicit_bytes' branch.