Fix image-replacement mixin invalid property

Turns out image-position doesn’t work with auto (and it was using the
string version anyways); default for image-position is 0, so I switched
defaults to that
Add icons for icon mixin + some cleaup

Add new icon names (search, cart, start, star-empty); add option to not
include &:before selector; remove icon content character codes for
subsetted version of FontAwesome (optimize at the end, not the
beginning)
Remove susy files and add the normal way

Adding susy as plugin (must be installed via `gem install susy`),
because it’s simpler and feels like the right way to do it