Use avocado state channel oracle to unlock actual ether

[jtremback]

Once #5 is resolved, we will be able to update channel state on the real blockchain. At this point, it's time to start looking at the actual applications of state channels.

Escrowing ether is outside of the scope of avocado, but we will need to think about how this will work. There will need to be another contract that actually holds the funds and releases them upon seeing that the state channel oracle has been closed and receiving the final state.

[jtremback]

We'll probably also want to use this contract to implement a hashlock, which should be a pretty trivial change.