Merge pull request #158 from apoloval/fix/BrokerSupervisor

Turn `BrokerSupervisorActor` into a service actor
Make broker supervisor wait for message gateway connection
Fix test broker initialization
Don’t use any key for the broker component
Merge pull request #159 from sortega/fix_it

Make broker supervisor wait for message gateway connection
Simplify BlockchainActor initialization
Merge pull request #160 from sortega/issue/225

Simplify BlockchainActor initialization
Make transaction broadcaster to receive micropayment channel notifications
Change from pull to push the last offer notification

From now on, the buyer micropayment channel will feed the transaction
broadcaster with the last signed offer.

This simplify the whole scheme and also, liberates the seller channel
from details it should not care about.
Fix step timeout handling in buyer channel actor

The timer was cancelled with any message instead of on step change.
Fix issue #240: handling of payment proof
Remove ConstantValueActor
Merge pull request #161 from sortega/issue/240_payment_proof

[Issue #240] payment proof handling