Till now, most of the modules were importing all of the module objects
(variables, classes, functions, other imports) into module namespace,
which potentially could (and was) cause of unintentional use of class or
methods, which was indirect imported.
With this patch, all the star imports were substituted with top-level
module, which provides desired class or function.
Besides, all imports where sorted (where possible) in a way pep8[1]
suggests - first are imports from standard library, than goes third
party imports (like numpy, tensorflow etc) and finally coach modules.
All of those sections are separated by one empty line.
[1] https://www.python.org/dev/peps/pep-0008/#imports
* bug fix - QR-DQN using error instead of abs-error in the quantile huber loss
* improvement - QR-DQN sorting the quantile only once instead of batch_size times
* new feature - adding the Breakout QRDQN preset (verified to achieve good results)
