Satoshi#to_s should print 8 digits satoshis

[speedmax]

Display small value in BTC is very hard in Ruby. cc/ @khoan

1. This should round small floats to 8 decimals, this happens when there is interest calculation on micro transactions

2. Satoshi#to_s shows small float instead of "0.00000320" 320 satoshis

> Satoshi.new(( interest  * micro_transaction ).round(8)).to_s
> "3.2e-06"