Add 'round' method to Money

The round method is handy when working at infinite_precision.
At some point you want to round the monitary value to cents.
This method is the explicit way to perform that rounding.
Write documentation for the Money#round method
Maintain the currency in Money#round
Rounding mode param for Money#round

You can now specify the rounding strategy to use when rounding.
Ensure fractional returns a decimal.

It was possible for the Money#fractional method to return
a Float with the combination of infinite_precision and
a deserialised Money instance.
Add Money#round method, bug fix for fractional method
Merge pull request #299 from orien/explicit_round

Add Money#round method