Bring back simple build/rebuild timer.

Adds the total milliseconds for a build to `Builder.prototype.buildTime` and is calculated at the end of a `Builder.prototype.build` run.

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.