Reduce withdrawal threshold and allow users to choose their fees #393

The withdrawal threshold appears to be hardcoded at 0.005 BTC. Note, however, that there are only two projects on https://tip4commit.com (this project and super3/peercoin.net) with a balance above 0.5 BTC. With a default 1% payment per commit, most contributors must get multiple commits approved in order to access their payments.

This is suboptimal and unnecessary. As I write this there are 567 transactions in the Bitcoin mempool totaling 0.2 MB. Historically speaking transactions with fee rates no more than 20 sat/b have rarely taken more than a day to get confirmed. Even a gnarly transaction combining lots of UTXOs (which anyway is less likely for smaller payments) is no more than 2KB. At 20 sat/b, that's a wildly overestimated fee of 40ksat, or 0.0004 BTC, 8% of the current threshold. Therefore I think it's a good idea:

1. to enable users to choose their own fee rate (perhaps with some nonzero minimum)
2. to set a threshold much lower than 0.005 BTC.