somtimes -> sometimes
if do -> if you do

Storing the message in args[0] seems more Pythonic.

While OAuthAuthorizationError inherited LoginError, they did not really share the same interface as they didn't receive the same args.

- Make `AssertUserFailedError` inherit `MwClientError` rather than `LoginError`, since it doesn't have the same args.
- Move message into args[0], which is more standard Python.

This is what is imported at the beginning of the examples.
This could also be done by changing the import to 'import mwclient'
rather than 'from mwclient import Site'.

Neither seems more correct but I have tried to follow
what seems most intended.

>>> from mwclient import Site
>>> site = mwclient.Site('en.wikipedia.org')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'mwclient' is not defined

vs.

>>> site = Site('en.wikipedia.org')
>>> page = site.pages['Greater guinea pig']
>>> text = page.text()
>>> page.exists
True

Use chunked file uploading for files larger than 1 MB as long as we’re on MediaWiki >= 1.20.

Python 3.3 is EOL, and is failing on Travis.

This fixes tests not being added to the source package.

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.

See https://github.com/github/markup/issues/556 for more details.

Bumpversion cannot update the release date and trying to remember to
update it manually won't work.

Attempting to fetch a login token will raise an APIError on older
MediaWiki versions (tested on v1.20.2), so we need to catch it.