Fix failing tests

Story, Product, and Image tests needed to test for the inclusion of
their relationships after those relationships were added.

For the Vendor model, at some point the 'id' field got accidentally
removed from the test. My guess is that it was in a merge conflict --
exactly what these tests were designed to prevent.
Begin vendor/product model join tests
Change tested model name to VendorProduct
Add __unicode__ messages to models, add to tests
Revert "Add __unicode__ messages to models, add to tests"

This reverts commit c232738a9ca56ab8b362b9c59792c72f5a65160d.
Add human-readable __unicode__ functions to models
Update __unicode__ functions, add tests
Update and move VendorProduct test
Test existance of filename method on Image model
Correct field types of vendor and products many-to-manys
Write vendor/product join model
Merge branch 'develop' into HEAD
Add docstring for VendorProduct model
Write VendorProduct model
Merge branch 'develop' into feature/16605
Fix Docs import errors