fix PageList API arg passing to GeneratorList

PageList wasn't adding the 'g' prefix to API arguments it
passes to GeneratorList, and GeneratorList doesn't do that
itself. This fixes it.

Adds six as requirement. It solves "ImportError: No module named six"

Conflicts:
	mwclient/listing.py

Quite silly, but until the problem is fixed on the server side,
it seems like this workaround is the best solution there is.

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

Back-ported to Python 2.6 and 2.7

Conflicts:
	RELEASE-NOTES.md

Filtering links by namespace not working

- 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

As suggested in #52
Original auth mechanism introduced in #27
This change should retain backwards compability

fixed link in user-agent section

reStructuredText is quite picky

Added instructions for changing default user-agent

Added instructions and an example for changing the default user-agent,
as in: https://meta.wikimedia.org/wiki/User-Agent_policy .

Use new token handling system described on
https://www.mediawiki.org/wiki/API:Meta#tokens for MediaWiki >= 1.24