Add rounding mode to configuration

Hi 

I've added a configuration setting for rounding_mode. Money seems to set this by default to ROUND_HALF_EVEN which in some cases rounds down 166.666667 rounds to 166.66 which was causing a weird bug for us. I imagine this is something that others would need to specify especially as later versions of Money now respect the setting more consistently - https://github.com/RubyMoney/money/pull/298

Thanks

Graham