Hey Kevin,

First let me say, this is a GREAT gem.  I'm absolutely loving it.

It would be really neat to have a quick way to say Unit.to_metric or Unit.to_english (or whatever other various systems
would be).  There might be a neat way to basically may one unit to another using a conversion hash or something.

I'll give you a sample usage.. I have a web site I'm building that displays all food measurements in gallons, pounds,
etc.  One of the first suggestions I got from users was to allow them to set base units in their preferences.  My canadian
people really want to see the metric system vs. our crazy english system here in the US.  It would be neat to have a
simple way to just convert all units without having to worry about what type of unit it is.