Print build times for `ember server`.

With the slow trees being printed automatically, we need some separation between slow-tree tables. It is also nice to see the total time to put everything in the proper perspective.

### Before

![before](http://monosnap.com/image/kt6E1zfAxIexTIW1t096Pe21G3FUwp.png)

### After

![after](http://monosnap.com/image/kF4G8QxWUQeXD8kgXuKBsGaaivXPxg.png)