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`.

In 1d617702 the Site.parse() command was changed from GET to POST. This
effectively limits the amount of wikitext that can be rendered to what
will fit in the URI limit of the webserver serving the MediaWiki site.
We could do something really tricky to look at the size of the text
argument and switch between GET and POST based on that, but it seems
much simpler to just stick with POST for this action.

Downstream bug: https://phabricator.wikimedia.org/T158715

Examples should adhere to PEP-8 guidelines, as these examples
not only document how to use correctly the client, but also
will serve as a first exposition to Python for some contributors.

Reference: https://www.python.org/dev/peps/pep-0008/

Allow sending custom headers (e.g. X-Wikimedia-Debug) with all requests
by passing a dict of header data in the Site constructor.

The issuestats service has always been quite fragile, and the badge was often a broken image.
Changing to isitmaintained badge, which (appropriately enough) seems to be better maintained.

fixes #142