Adding factory-boy package for creating easily fixtures for tests

[factory_boy](https://factoryboy.readthedocs.org/en/latest/index.html) is a fixtures replacement for tests.

Its features include:

* Straightforward syntax
* Support for multiple build strategies (saved/unsaved instances, attribute dicts, stubbed objects)
* Powerful helpers for common cases (sequences, sub-factories, reverse dependencies, ...)
* Multiple factories per class support, including inheritance
* Support for various ORMs (currently Django, Mogo, SQLAlchemy)