Update popit-python for the current version of PopIt

Since `popit-python` was last worked on there have been a number of
changes to popit-api, including:

* The version number changing to v0.1
* "Organization" now being spelled with a 'z'
* Collections now have a plural name (e.g. `persons` rather than `person`)

This pull request makes changes related to those issues, and also renames
the module from `popit` to `popit_api` to avoid a clash with `popit-django`.