Adds isStandard risk analysis engine and allows cleaning up the wallet.

For now, it only checks for dust spam. All other checks have been split into https://github.com/schildbach/bitcoinj/tree/stdtxnriskanalysis-next

I have added more unit-tests and also tested this with the real app. Everything works as expected, both cleaning up and protection against "re-infection".

If you merge this to master, I can cherry-pick it as-is.