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

Work in progress, looks a bit dirty right now

Conflicts:
	mwclient/client.py

Also replaced relative links in the README, since PyPI doesn't support
them, see http://stackoverflow.com/a/16594755/489916

- This is more flexible than the `Page.get_expanded()` method,
  since it can be combined with the `section` argument as well,
  to get the expanded wikitext of a section.

- Deprecating `Page.get_expanded()`.

- Also fixed a very minor bug where `Page.last_rev_time` was
  not reset when editing new pages after having edited non-new pages.

- Added more Page tests

Remove more version checks that are no longer needed

The missing comma would cause site.images[filename] to fail

This class provided workarounds for editing pages without 'writeapi'.
Since 'writeapi' has been enabled by default since MediaWiki 1.14,
and we require MediaWiki 1.16, it should no longer be needed. If
'writeapi' has been explicitly disabled, we should rather raise an
error, since this means that automatic edits are not welcome.

since the method does not edit the page, but returns the text of the
page. The old method name still works, but issues a DeprecationWarning.
Note that DeprecationWarnings are silent by default, so we might
consider to elevate it in some future version.

Since we now require MediaWiki 1.16, these are no longer needed. It
might be necessary to add new compability methods in the future, but the
API is generally stable now.

Replace 'http.py' with the Requests lib

If we do change it later, then we can update the readme accordingly.