This is a wide-sweeping change which allows the Python client to run on these versions:

• 2.6.9
• 2.7.8
• 3.3.6
• 3.4.2

The six module is critical in supporting both Python 2 and Python 3 and is used whenever appropriate.
In other places conditions have been added with the six.PY2 and six.PY3 constants to run the appropriate code.

Here are a few changes from Python 2 to 3 which were addressed:

• print is now a function
• Relative module pathnames are no longer supported
• All strings are now Unicode by default
• PBC library requires binary (bytes) instead of strings
• dicts return iterators, not lists in iterable contexts
• long type has been deprecated and int is now 64-bit
• Libraries have been reorganized, especially HTTP and exceptions, so imports have changed
• next() in the iterable interface is now __next()__
• Socket library is greatly different, so for security the native ssl library is used instead of PyOpenSSL to support HTTP
• Python 3.3 ssl library is not as mature as Python 3.4 so lacks a few features (like TLS 1.2)
• Python 3 ssl library only allows certificate files, not bytes treams, to be added so the SecurityCreds class does not support cached files like it does in Python 2
• Some unit test assertions were renamed, so test_six.py was introduced to hand the renames
• xrange() was retired so range() was used in its stead
• Tests for riak_object were adjusted in Python 3 since strings need to be encoded before sent to the transport layer
• The built-in unit tests in Python 3.4.2 actually don't test CRLs, so for now they are not supported in Python 3.x client
• Python 3 client supports Unicode key and bucket names, unlike Python 2
• The file open interface changed slightly

NOTE: This PR is dependent upon riak_pb basho/riak_pb#104