moved css compilers into their own module
removed accidentally commited built test files

Also added the whole test build folder to gitignore
added option for separate retina style sheets

If the option 'splitHighResAssets' is set, stylesheets are compiled one
extra time with resolution set to '2x' on the css compiler. It is up to
the compiler to actually make use of this flag and produce a stylsheet
that is meant for retina devices.