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.

Retry on nonce errors since these can be caused by temporary cache connection errors.

When there's zero results, the method should not yield an empty
dictionary. Otherwise counting number of results could be wrong (think
`len(list(site.ask('...')))`)

The method only returned dictionary keys, not values.

Misplaced parenthesis

On read protected wikis, we must make sure that we don't include extra
parameters when requesting the login token, or we will get
`readapideniederror`.

- Remove extraneous `continue` parameter from non-query calls
- Remove `userinfo` from `meta=tokens` calls

Update the `Page.purge()` method to use the `action=purge` API module
that was added in MediaWiki 1.14. Since mwclient requires MediaWiki 1.16
or later, we don't need to also support the old purge method.

For MediaWiki >= 1.27, fetch login tokens from the tokens module
using `action=query&meta=tokens`.