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