Entity Object __unicode__ method and tests.

@wesleykendall This `__unicode__` method does the following:

1. Checks if `settings.ENTITY_NAME_KEYS` exists. If not, it defaults to `('name',)`
2. For each potential name, it checks if there is a corresponding key in `entity_meta`, if so, return the value
3. If nothing is found, return `'Entity Object'` as the unicode representation.