Refactors addFourValueShorthand to accept hash of options.
Shortens `border-radius` addFourValueShorthand calls.
Removes extra spaces around / in border-radius.

* Apparently horizontal & vertical values can be separated by '/' without spaces around - thanks @XhmikosR.
Replaces same horizontal & vertical value in border-radius.

* border-radius:1em/1em can be turned into border-radius:1em.