Back-ported to Python 2.6 and 2.7

- 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

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.