Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions

* homescreen: px to rem, support 5 rows of icons, small tweak in e.me page
* added @2x assests for homescreen

There are some JS changes to allow having 5 rows of icons and also for rendering icons at the proper size (this one is only needed if we do not have scale support from gecko).

r?

@timdream @jmcanterafonseca @rexboy7 @patrykdesign @sergivila