Add communication between actors and main

Multiple actors will have to communicate between them by sending
transactions to each other so a prototype of this communication is
created here where the main goroutine plays the part of a coordinator.
A OnBlockConnected notification has been added to notify main to stop
transcations when a second block, after the actors have started running,
has been added in the blockchain.

Other than that, almost every use of log.Fatalf has been dropped to
log.Printf and use of a time-dependant random seed has been added in favor
of purely random address selection inside actors.

The following changes have been tested on ```testnet``` with lines 217 to 225 of
```actor.go``` commented so actors can actually get in the ```select``` statement.