Go bindings and tests

[shanev]

• auto-generate bindings whenever Solidity file changes using go generate
• tests using blockchain simulator

[bonedaddy]

One thing to keep in mind about the blockchain simulator, I have never ever been able to get the adjust time function to work from go-ethereum's simulated backend:

https://godoc.org/github.com/ethereum/go-ethereum/accounts/abi/bind/backends#SimulatedBackend.AdjustTime

[shanev]

@postables Ah that sucks. We may not have to use it if we go with block height vs. time for the HTLC.