Added tell supervisor action to make system init work

Fixes #16 by introducing a new tell supervisor action, that is sent additionally to the ActionJoinAnswer message.
If the answering node does not know the supervisor yet, it will postpone the "tell supervisor" action until it does know one.
That way, we are able to spawn large amounts of wallets

See merge request !4

#14: delete unused files like Wallet.java, move methods of WalletControl.java interface to AbstractWallet.java and drop WalletControl.java