Allow for shorthand props containing strings or colors, plus remove extra whitespace 

Hi there,

I found your mixin from this article http://webdesign.tutsplus.com/articles/typography-articles/taking-ems-even-further/. I really like it, but made a small edit so that you can convert shorthand properties like border, which contain numbers and strings, to rems. I also made a minor update to remove the extra leading whitespace before the values in the generated css.

Thanks for posting this mixin!

Ted