Feature Request: Reward for hitting mob

[OfriHarlev]

Ability to give reward for hitting mob

[DaveyBiggers]

Closed in #545.
See hit_test.py sample for a demo.
Can assign different rewards to different entities, and also use the usual reward-distribution stuff - should be fairly flexible.

Note that the reward only counts if the entity's damage source was the player - ie if a pig dies because the agent pushed it into lava, the cause of damage will be lava, not the player, so the player won't get the reward.