show time it takes to compile Stylus file (bug 1091421)

notice that the times are not accurate for the files compiled when `make serve` starts up. but the times look good when Stylus files get rebuilt during the file watching.

and I know I'm not doing this correctly. guidance please, @ngokevin? thanks.

![screenshot 2014-10-30 22 28 45](https://cloud.githubusercontent.com/assets/203725/4856642/27527876-60bf-11e4-8fef-15913a52a362.png)
![screenshot 2014-10-30 22 25 47](https://cloud.githubusercontent.com/assets/203725/4856643/27579ee6-60bf-11e4-9ccc-1484251247cb.png)