changed the String() method for UUID's

I was profiling some code for work, that uses the UUID.String() method a lot and saw it was using fmt.Sprintf(), which is a bit overkill, if you ask me. I changed it to use a switch statement. It is now between 4-6 times faster. Here is a [benchmark](https://gist.github.com/cwandrews/75a2b8f92d96e2aeb083).