Improvement to number_to_phone to handle more countries

My app needed a good way to format a phone number for countries other
than the US. So I added the "groupings" option to the
number_to_phone() helper. I'm not sure if this will allow the helper
to accommodate all countries, but it will go quite a way further than
it does now.

I'd love to get some feedback on it, and possibly inclusion in Rails
core. Thanks.