docs: older pip might install pre-releases

Looks like pip might install e.g. 2.5.1-rc1 when 2.5.1 has been
released.  It would seem that their version detective work isn't working
in those cases.

This occured on Debian Wheezy (7.4) and pip 1.1

I don't think we want to remove the old versions from pip because we
want to allow fallback path for users if required and because these are
FOSS releases so we shouldn't just remove them.

So make a note in the install guide for this possible occurance.

Fixes bug 3185 to some extent.
Document changes in custom CSS handling

Since e95286d90c8d0077ccc1117021c3856a628c0ef2 the custom.css file is
bundled with the main file, URLs are now relative to the parent css
directory.

Conflicts:
	docs/developers/customization.rst
The convention is to use cls for first parameter in classmethods
Bugfix: Change reference to unexistent cls object

Use self instead to access the classmethod.
Black magic that fixes Evernote issue #4

If we retrieve the actual SQL query then the Paginator breaks for
querysets containing `.extra()` calls. This DIRTY HACK allows the actual
`SELECT SUM (*) FROM ...` SQL query to select and join the necessary
tables.

(I suspect that the current filtering constraints aren't fully populated
when the paginator tries to calculate the count, and this hack somehow
forces it.)

This might be a bug (or undocumented behavior) in Django's ORM querying
facility internals, so any further investigations should be done in that
direction.

Original issue is at evernote/pootle#4.