UPDATE If set, BaseCurrencyPrecision should be used in Order processing

Prior to this commit number_format in Order::process is hardcoded with a precision of 2. As previously, the default precision will be two, and only changed if BaseCurrencyPrecision is set in the ShopConfig.

NOTE: A current limitation of the Silverstripe framework is that the Money DBfield uses Decimal(19,4) for the Amount, which is outside the scope of Swipestripe and the Payment modules. I'm creating a pull request to address this;