Python 3.3 is EOL, and is failing on Travis.

This optimizes setup.py for cases when pytest-runner is not needed,
using the approach that is suggested upstream:

https://pypi.python.org/pypi/pytest-runner#conditional-requirement

This reverts commit 1650e0e7.
Upstream restored handling of the adding_headers arg in 0.6.1.
Going back to using that name means we're still compatible with
any version of responses after 0.3.0, except 0.6.0. Also adjust
setup.py to express this correctly.

Remove python2.6 from
- Travis
- Tox
- PyPi tags
- documentation and readme

Also, Simplify an import clause.

This fixes #133

Looks like a buggy pip version could be the root cause, but
nevertheless, we shouldn't need to specify that we're depending on
requests explicitly when requests_oauthlib depends on it.

(https://stackoverflow.com/questions/27497470/setuptools-finds-wrong-package-during-install)

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.