Add third argument to FontFace contructor, fix #2

This argument, `FontFaceDescriptors`, is optional according to spec but
causes some browsers (Chrome 35 and 36, Opera 22 and 23) to throw an
error.
Merge pull request #3 from CrocoDillon/master

Add third argument to FontFace contructor, fix #2
Merge branch 'master' into gh-pages