Bring back simple build/rebuild timer.

Does not offer nearly the same ability that was removed in
https://github.com/joliss/broccoli/commit/0fcb34b2ec0a8fe22720bb597796ba5e0d705757,
but it is useful to generally know if your build time is over/under some
reasonable number.
Merge pull request #111 from rjackson/bring-back-crude-timer

Bring back simple build/rebuild timer.