Add mining

Mining has been added with the use of the NewMiner function so now a
complete simulation is possible to happen. The way NewMiner works is
that it shutdowns the initial btcd and replaces it with a cpu-mining
enabled btcd relying on the fact that btcwallet instances, when disconnected,
will constantly try to reconnect back to the same port.

Also the communication between actors and the main goroutine has been
upgraded to accomodate actors with insufficient funds.