Fix legal_actions_mask bug in epsilon_greedy().

[jhtschultz]

This PR addresses Issue #27

Note that the bug must be addressed in two places. First, when selecting max_value - it must only be selected from legal actions. Second, when computing greedy_probs - there could be multiple action values achieving the max, but not all of them legal.

Also, I added legal_actions_mask to the list of values in the tf.name_scope context manager.

Aside from that, when there is no legal actions mask the epsilon_greedy function should execute exactly the same as before.

Happy to implement any small changes and if there’s a better fix altogether feel free to close this and implement it internally. Just thought I’d offer up a solution :)