Add OAuth and update docs

- Add zip_safe flag

Prepare for Python 3.x support
- Use print function
- Use compability methods from six

- If database lag exceeds max lag, the API will still send 200, not 503.
  This was only added to documentation after bugzilla:31156.
  Ref. <https://www.mediawiki.org/wiki/Manual:Maxlag_parameter>
- In addition, Requests doesn't raise an exception when status code != 200
  unless raise_for_status() is called, so we should call that to catch
  any other 5xx errors such as Squid errors.
- Replace print statements with logging

Note that responses>=0.3.0 is needed to run the test suite now

Also replaced relative links in the README, since PyPI doesn't support
them, see http://stackoverflow.com/a/16594755/489916

- Require 'ordereddict' only on Python < 2.7
- Don't require 'simplejson'

- For some reason, 'pytest-cache' isn't found by setup.py if not
  specified explicitly in 'tests_require'
- Add tox.ini so tests can be run using tox if desired
- Update test to not be dependent on serialization

- A single command `python setup.py test` will now install test
  dependencies, make an in-place build and run the tests.
- The setup script now requires Setuptools. A message will be printed
  if it is not found.

- Note the following exception replacements:
  mwclient.errors.HTTPError -> requests.exceptions.ConnectionError
  mwclient.errors.HTTPStatusError -> requests.exceptions.HTTPError
  mwclient.errors.HTTPRedirectError -> requests.exceptions.TooManyRedirects

- Removed the `compability.old_upload` method, which was used for
  MediaWiki < 1.16.

- Requests automatically decompresses gzip-encoded responses, and
  does its best to decode response content to unicode when possible.

- [#53] Using the Responses library to mock http requests in tests