Add prioritized experience replay

[r7vme]

Right now policy learning process is vague and does not improve over time (i.e. good policy can be learned in 3 episodes, but after 10 episodes policy can degrade completely).

So after spending time adjusting optimal buffer size and optimization steps, i see that it's pretty random. I assume that prioritized experience buffer for DDPG will help imrove situation.

In short, it will make sure

• to sample unseen observations (by using infinite priority)
• to sample "valuable" observations (by computing priority based on TD error)