Added Black-box alpha inference method, added unit test

[willtai]

I want to add black-box alpha as an inference method. I have added the BBAlpha inference object and the unit test bb_alpha_test.py. Please do help to make my code better or give suggestions.

Here is the link to the Edward forum: https://discourse.edwardlib.org/t/black-box-alpha-divergence-minimization/413/5