Using closure to compile underscore-min.js

It's basically a copy of backbonejs' Rakefile.
I saw that @jashkenas changed it on the last release but uglifyjs was always outputting the result, so, in order to make it as smooth as backbone's building process i copied it. Plus, there was a -m option in the uglifyjs command which i couldn't find its purpose.
This is more like a polite request than a change/fix. I tried to find previous versions where it happened or old tickets about the process but found nothing. Sorry if i missed something and went back in time with something already discussed and closed.

Cheers.