add maxAmount and minBlockDistance setting from simple-wallet dApp

[bitgamma]

In a web3 browser, allow the user to set the maximum transaction amount. Also allow the user to set the cool down between two payments. This is expressed in the contract in number of blocks, but knowing the block time the user can be presented with an interface showing approximate seconds/minutes instead.

Since the procedure is different depending on whether the owner of the wallet is the Keycard Cash applet itself or another EAO (ideally, the Keycard Wallet) it could be a good idea to activate this functionality only after the user moved the ownership of the wallet away from the Keycard.