Add support for Python 3

... at least for the code run by the tests on Travis CI.

https://travis-ci.org/hugovk/twitter/builds/23027808

Notes:
 * 2.6, 2.7, 3.3 pass.
 * 3.4 isn't quite yet available, but is coming soon: https://github.com/travis-ci/travis-ci/issues/1989#issuecomment-39315535
 * 3.2 fails because it doesn't have u'uncode stuff' but I guess most Py3 users will be ok with 3.3 or 3.4.
 * I've not looked into why pypy fails.
 * (This PR also contains the same changes as PR #220.)