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.
这里 `self[:quick_withdraw_max].to_s` 如果 currencies.yml 中没有设置 quick_withdraw_max 的时候，要把 nil 转换成空字符串么？如果用 `field :quick_withdraw_max, default: 0` 来设定默认值是否更清晰呢？