Fixed two kerning-related issues. Firstly, Paper.registerFont was checking for
whether or not kerning was specified on the font correctly, so it wasn't
being read in. Secondly, the calculation for the kerning in Paper.print
wasn't setup correctly - it was adding the kerning value from the previous
letter spacing instead of subtracting it. Changing it to subtraction made
things line up with how they're displayed in Illustrator.