Add support for randomized "dummy payments" on the wallet CLI

[riemann89]

In testEtherMixing.py and testERCTokenMixing.py add dummy payment from Alice to Alice where sum inputs = 0 = sum outputs (payment that is only used to make noise).

[AntoineRondelet]

We can also improve the wallet by allowing "randomized dummy payments to self", to add noise to the system at the expense of paying for the gas cost of the smart contract call. The idea would be for the wallet to run, and spend - at random interval of time - 0-valued notes to create other 0-valued notes for the user of the wallet.