Decouple snap from messaging and from routing

[michielbdejong]

Snap should be a stand-alone implementation of a bilateral ledger that requires:

• a way to send a message
• a way to receive a message
• a way to persist data (key-value store?)

And that offers:

• a way to propose a ledger entry
• a callback to accept/reject an incoming proposal
• a way to query the balances
• a way to set min/max limits on those balances, such that attempts to go over them will fail.

[michielbdejong]

Now created a half-ledger class.