Fix rendering on iOS 7 devices
Fix rendering on iOS 7 devices

This fixes #5

The issue ended up being that UIFont's fontWithName:size: method
handles nil arguments differently on iOS 7. On iOS 6 passing nil
for the name meant the method would return nil, here it returns
Helvetica. As the size here is also nil, we ended up getting 0pt
Helvetica.

rpowelll/RPSyntaxHighlighter,rpowelll/RPSyntaxHighlighter