This fixes tests not being added to the source package.

- Most importantly, the 'protectedpage' error was missing in the list.
- Improve string representation of the error.

- When `Site.force_login` is True we can send `assert=true`
  with `edit` actions to have the api reject our edits if
  we got logged out for some reason.
- Adding a new exception `AssertUserFailedError` as a subclass
  of `LoginError`.

New implementation of #126

- Remove `Page.get_expanded()`
- Postpone removal of `Page.edit()` since it's a more popular method.

per #130

Store the results of page.text() operations in a simple cache
dict. This avoids unnecessary remote roundtrips. Cache is
cleared on each successful page.save() operation. cache
argument can be set to 'False' to disable use of the cache.

Prepare for Python 3.x support
- Use print function
- Use compability methods from six

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.