More Data Models

[chrisvanrun]

This PR contains some extracted (or more re-achieved) changes from:

• Major refactor of helper functions #167

When I was doing a major re-haul and unwisely decided that, since I was touching nearly everything, it made most sense to push the changes upon the already large change set.

This PR does two major things:

• Allow object/model retrieval using api_url (was done with the direct request, now forced through the APIs)
• Fix resulting deprecation warnings on key access (i.e. object["pk"]

It also makes the recurse_call wrapper more robust by outputting the error (raise from e) and allowing for retries when ObjectNotFound is thrown.

Widespread changeset, but relatively simple changes.

[jmsmkn]

Looks good, small comments.