Added uuid.Must()

Hello,

I've added a Must() function to help while initializing variables. Inspired by  [text/template](http://golang.org/pkg/text/template/#Must).

I'm not sure how often a `NewV4()` produces an error, but from what I can see `NewV5()` never does. Please correct me if I'm wrong. 

`uuid.Must()` should be used with caution of course, as it forfeits  the ability to handle an error.

- Alex