Fixes #8, an issue with proxy rule with just a '/'

Really, a proxy rule '/' should be empty string, as we do not want to replace anything. This
commit treats a '/' in that way.
Use sass css compressor instead of yui (fixed #9)
Switched to UglifyJS for JS compression

UglifyJS does as well or better than YUI on every example I've found.
It's also much faster, as we no longer need to start a JVM.
Don't recompile all files when something updates, just redetect directives
Add tests for slinky_depends
Don't compile SASS partials