Working GAIL, pretrain RL models and hotfix for A2C with continuous actions

• fixed various bugs in GAIL
• added scripts to generate dataset for gail
• added tests for GAIL + data for Pendulum-v0
• removed unused utils file in DQN folder
• fixed a bug in A2C where actions were cast to int32 even in the continuous case
• added addional logging to A2C when Monitor wrapper is used
• changed logging for PPO2: do not display NaN when reward info is not present
• change default value of A2C lr schedule
• removed behavior cloning script
• added pretrain method to base class, in order to use behavior cloning on all models
• fixed close() method for DummyVecEnv.
• added support for Dict spaces in DummyVecEnv and SubprocVecEnv. (@AdamGleave)
• added support for arbitrary multiprocessing start methods and added a warning about SubprocVecEnv that are not thread-safe by default. (@AdamGleave)
• added support for Discrete actions for GAIL
• fixed deprecation warning for tf: replaces tf.to_float() by tf.cast()
• fixed bug in saving and loading ddpg model when using normalization of obs or returns (@tperol)
• changed DDPG default buffer size from 100 to 50000.
• fixed a bug in ddpg.py in combined_stats for eval. Computed mean on eval_episode_rewards and eval_qs (@keshaviyengar)
• fixed a bug in setup.py that would error on non-GPU systems without TensorFlow installed

Welcome to @AdamGleave who joins the maintainer team.