add custom exponent fuction

we can use this for our custom configs[11/19/14, 12:23:06 PM] Danny Yang: it's to allow us to easily change any configs in money easily, and speed up the calls
[11/19/14, 12:23:13 PM] Danny Yang: [7] pry(main)> t = Time.now;100_000.times{Money::Custom.exponent('usd')};Time.now - t
=> 0.054287
[8] pry(main)> t = Time.now;100_000.times{Money::Currency.new('usd').exponent};Time.now - t
=> 0.38837
[11/19/14, 12:23:52 PM] Danny Yang: in mapi, we have the '%.2f' and '%.8f' hard coded
[11/19/14, 12:24:09 PM] Danny Yang: better to use Money gem's config