Issue 855989 CSS optimization in build process

The patch will monitor html, aggregate and minify css files in /style folder to style/gaia_build_<html>.css

aggregated styles must within 'style/' folder because its relative to image resources.


* optimize css in build process with modified clean-css https://github.com/GoalSmashers/clean-css
* aggregate css in /style folder to keep relative resources path
* not aggregate share styles since its packaged in webapp-zip and each share style is located in its subfolder, its not trivial to reference every resources without changing the css definition.
* separate CSS optimize from JS for performance evaluation
* keep linebreak for debug