#10 bug in multiplication of UOMs

nasty bug in UOM.asPrimeMap. When adding two maps together it could lose
 data from the second map.

 Fixes #10
Odd that I get to be the voice of scalaz restraint but perhaps a more explicit use of scalaz would be clearer, later if you end up using it a lot it will be more familiar to everyone and refactoring to use the syntax pimping will be a smaller 

Also I can't tell which semigroup is being used for the values, the additive or multiplicative ?