Add rake task to build with closure advanced optimizations.

You would use this to compile your code _with_ underscore, to
remove all those dead underscore functions you don't use.

Slight tweak to code to `root._` code to get working, and must
remove anonymous wrapper function for current closure compiler to
remove dead code.
(cherry picked from commit b5449ca89a6defbe78196b2686a2131bf18eb751)