New features to add: PolicyGradient, OpenAI Gym, Keras, Asynchronous Training

[lsqshr]

Since I am using this repo in my work as well, some of the features I found might be useful:

1. Continuous control: A couple of recent papers have been published to show that deep NN would enable complex high dimensional controls with policy gradient. It seems the continuous branch was initiated but has been inactive for a while (is it totally abandoned?).
2. Support OpenAI gym to make continuous control easier and bench-marked
3. Support Keras to shrink the code base regarding to tensorflow, also make it easier to program more complex networks
4. Asynchronous training with multi-agent

The latter three are supported in Asyncronous RL in Tensorflow + Keras + OpenAI's Gym However I do prefer the architecture here to keep things well structured.