Show detailed rebuild timing information.

Uses the same logging as Broccoli itself. By default it logs any trees that took greater than 5% of the total build time.

Also, we now print `Built Successfully - XXXms.` for each rebuild to assist in tracking down performance issues. We may decide to remove the more verbose logging at a later time once things are better understood (and we have near constant time rebuilds).

![screenshot](http://monosnap.com/image/PPiU7NIqs4xkIAMrsVGPtXQkpKrnd4.png)