Move demos app

This is just a straight move with the only style changes being to fix up the imports; ``kuma/demos/__init__.py`` really ought to be killed off, but that's a refactor for another day.
PEP8 says:

    standard library imports
    related third party imports
    local application/library specific imports

I also separate the related third party imports in non-django and django like you, @groovecoder and there is precedence in my experience.

That said it seems as if the webdev group does it slightly differently (http://mozweb.readthedocs.org/en/latest/coding.html#import-statements):

    Separate imports into groups with a line of whitespace:
    standard library; Django (or framework); third-party; and local imports