0.5.1: Full Mujoco env support

• Ant-v4 (with @yufansong #74)
• HalfCheetah-v4 (#75)
• Hopper-v4 (#76)
• Humanoid-v4 (#77)
• HumanoidStandup-v4 (#78)
• InvertedDoublePendulum-v4 (@Benjamin-eecs, #83)
• InvertedPendulum-v4 (#79)
• Pusher-v4 (#82)
• Reacher-v4 (#81)
• Swimmer-v4 (#80)
• Walker2d-v4 (@Benjamin-eecs, #86)