Fix implicit int->unichar conversions on building character sets

At some point, these character sets should be extracted into category methods on `NSCharacterSet` (especially `newLineAndWhitespaceCharacters`, which is duplicated in this code). I may take this on this weekend, but I don't have time at this moment.
Merge pull request #4 from cdzombak/cdz/fix-implicit-conversions

Fix implicit int->unichar conversions on building character sets