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 the someBigNumber-th 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, use of a time-dependant random seed has been added in favor
of purely random address selection inside actors and method BalanceUpdate
has been added for Actors.
Declare this in the const block above.