Returns a random v4 UUID of the form
xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx, where each
x is replaced with a random hexadecimal digit from 0 to f, and
y is replaced with a random hexadecimal digit from 8 to b.
Instance, based on recursion, can be found here.
nice work, @LeverOne. it's 1 byte shorter and faster too: http://jsperf.com/uuid-golf