Use the getattr function rather than the internal __getattr__ method

I don't think there's any reason to use the internal method here.
Update to support paginated responses from popit-api

Versions 0.0.14 and later of popit-api return collections paginated
by default; this commit updates popit-django so that it will fetch all
of the results whether the results are paginated (version >= 0.0.14)
or not (version < 0.0.14).  Previously, only the first page of results
would be fetched.

Fixes #21
Factor out the code for getting possibly paginated results

By factoring out the code to get results from the API even if they're
paginated, we can make it accessible to other clients.  For example,
even though popit-resolver uses popit-django, it still uses get() on
Slumber resources without taking pagination into account - instead it
can now use get_paginated_generator instead.