Merge pull request #91 from sortega/task/134_keypair

Take a new key pair from the wallet actor for the exchange
Add messages to block and unblock fiat funds
Move AnyPayment to model module
Extend OkPayClient trait

Allow getting all the balances with just one request
Make OkPayProcessorActor able to block/unblock funds
Make PaymentProcessorActor use blocked funds for payments

Every payment should refer to previously blocked funds to avoid
exchanges stepping into each other.
PR feedback
Merge pull request #92 from sortega/task/131_fiat_funds

Manage blocked fiat funds
Use the real payment processor id when starting exchanges
Use ImmutableTransaction in WalletActor
Use WalletActor instead of a bare Wallet
Merge pull request #94 from sortega/task/135_integration

[Integration of DefaultExchangeActor]