having dec2hex in the function scope give a little of a speedup

[marianoguerra]

here is the jsperf

http://jsperf.com/uuid-with-local-vs-upper-scope-dict

[marianoguerra]

avoiding the casting of the number 4 on the string concatenation seems to help too

http://jsperf.com/uuid-with-local-vs-upper-scope-dict/2