Test to check the dict (and nested dictionaries) can be JSON serialized. This test fails.
UserDict - no reason was given for reimplementing a core Python data structure. Furthermore, the custom AttrDict doesn't JSON serialize so an object built from JSON cannot be serialized back to JSON.

**BREAKS BACKWARD COMPATIBILITY.** This is now a real Python dictionary and *no longer has attribute access* for the data. This could be added back, but it's un-pythonic and introduces complexity. UserDict retains the ability to access `data`. The `dict` property is gone.
Add a test to ensure that object attributes don't collide with dictionary data.
Merge branch 'use_dict_not_custom_class' of https://github.com/joncotton/embedly-python into joncotton-use_dict_not_custom_class

Conflicts:
	embedly/models.py
	embedly/tests.py