Integrate SATP with BUNGEE

[RafaelAPB]

SATP will now use BUNGEE to create the proofs sent to counterparty gateways.

Requirements:

1. Define a mapping of relevant information that a SATP proof must have (eg for private blockchains a view including the changed state is enough; for public blockchains a view including the state change + transaction hash, for instance)
2. At the stage when a proof of a transaction is created, call BUNGEE plugin instead with the corresponding arguments relevant to that transaction.
3. Map BUNGEE response to proof object, and send the proof to counterparty gateway

[eduv09]

@RafaelAPB please assign me this one