A cent is the smallest unit we can deal with.
Properly implement per-user limits.

We only had a limit for invididual transactions before. The daily
limits are per target bank account, since at quoting time we don't
actually know the Ripple address sending, which would make a source
limit hard to implement. Ripple accounts are also much easier to
come by than bank accounts.

Renamed the limit config vars in the process.
Two passing tests were passing for the wrong reason.
Make the advertised rates dynamic.