* Allow arbitrary dimensional observation (non vector or image)
* Added creating PlanarMapsObservationSpace to GymEnvironment when number of channels is not 1 or 3
* Changes required for Continuous PPO Head with MXNet. Used in MountainCarContinuous_ClippedPPO.
* Simplified changes for continuous ppo.
* Cleaned up to avoid duplicate code, and simplified covariance creation.
Adding mxnet components to rl_coach architectures.
- Supports PPO and DQN
- Tested with CartPole_PPO and CarPole_DQN
- Normalizing filters don't work right now (see #49) and are disabled in CartPole_PPO preset
- Checkpointing is disabled for MXNet
