Initial work to support Python 3.3+

This updates most of the code to be forward-compatible with Python 3.3 and
3.4 while still continuing to support 2.6 and 2.7. It **drops** support for
Python 2.5.

Python 3 support is added for common Boto modules (`boto/*.py`) as well as
S3, SQS, Kinesis and CloudTrail. Several other modules may work but have
not been thoroughly tested.

The `tox` configuration has been updated to run tests for all supported
environments, and for now a whitelist is used for Python 3 unit tests.

A new porting guide is included to help community members port other
modules to Python 3, and both the README and Sphinx index list which
modules currently support Python 3.