Merge pull request #113 from apoloval/task/138

Make exchange actor to use the funds blocked in payment processor
Add another signature for blockMultisignFunds

This allows for handpicking the inputs of the multi-signed transaction
to be created.
Implement bitcoin funds blocking
Merge pull request #114 from sortega/task/132_block_btc

Implement bitcoin funds blocking
Rename FundsId -> BlockedFundsId
Rename CoinsId -> BlockedCoinsId
Move BlockedCoinsId to the model module
Introduce bitcoin blocked funds on Exchange description
Allow to subscribe on wallet changes
Introduce OrderFundsActor

It is able to manage both fiat and btc funds of and order.
Merge pull request #116 from sortega/task/132_use_btc_blocking

Prepare for using bitcoin funds blocking
Change payment processor actor to retrieve detailed balance info

Retrieving only the total funds as payment processor balance is naive. Since we
are blocking funds, and the operative depends not in the total funds but the
available ones, the payment processor actor should be able to retrieve detailed
balance info to the requester.
Change `CoinffeinePaymentProcessor` to retrieve detailed balance

According to previous commit, the defailed balance is managed by
`CoinffeinePaymentProcessor` as well.
Replace `NoFundsBlocked` by `BlockedFunds(0)`
Merge pull request #117 from apoloval/task/OrderSubmissionPreconditions

Retrieve detailed balance from payment processor