Inject collaborators in DefaultExchangeActor to use the BTC network

In order to make DefaultExchangeActor to broadcast transactions, it should
have a TransactionBroadcaster object at hand to do that. This change prepares
the actor to receive such collaborator as a dependency injection.
Define the messages for transaction publishing in BlockchainActor

Define the case classes that describes the messages used by a blockchain actor
in order to publish a transaction to the Bitcoin network.
Implement transaction publishing messages in DefaultBlockchainActor

Implement the handling of transaction publishing messages in
`DefaultBlockchainActor` class. The corresponding unit tests are included.
Merge pull request #213 from apoloval/issues/90

Implement transaction broadcasting feature in BlockchainActor