Automatic client challenges by watching exit events

[aupiff]

Next step is to figure out how to store all pertinent coin history on client. This will be easy enough if at first we naively scan all of the plasma chain to gather coin info BUT each client does need to be woken up whenever they are sent a coin (In order to implement this feature, we'd have to decide on a p2p protocol). @gakonst let me know if this sounds good.

• added exit-watching functionality
  • created & tested function to allow users to stop watching coins as well
• added basic challenge framework in callback to StartedExit event
  • fetches exit information from root chain at start, then decides what type of challenge is appropriate

Question: should pushExit be its own function? It's only called once. We might save a good amount on gas if we rolled it back into startExit.

[gakonst]

Added automatic challenging for challengeAfter and challengeBetween.

[aupiff]

LGTM, @gakonst you should merge.