auto approve withdraw if the sum is under certain amount

New feature, check https://github.com/peatio/peatio/issues/191 for more details.

To enable auto approve, add `quick_withdraw_max` setting under correspond currency in  `config/currencies.yml`.

By default quick withdraw is disabled.
nil是一个原因，但更主要的原因是`quick_withdraw_max`可能是个float, 而`BigDecimal.new`只接受string, 所以即使用了`field ... default`, 这一行还是要这么写.