Have the icon.size default to computed font size.

This is a proposal to have the icon size default to computed font size. Very often, icons are presented next to text of matching size. This would make it possible to set the font size with CSS and have the icon automatically resize. Does this sound useful?
Parsing this float so that we can convert it back to a string on line 109 seems like a bad idea. Not using `this.size` at this point might solve the IE units problem mentioned above too (if that problem still exists).