Implement correct priority of "-" in format strings under 1.9

Ruby 1.9 and 2.0 give higher priority to the `-` (left justify) operator than to the `0` (pad with zeros) operator when used in format string in conjunction with numeric values.

See http://ujihisa.blogspot.com/2009/12/string-differs-between-ruby-18-and-19.html for details.

This pull request fixes the two failing RubySpecs by extracting two methods out of the `Sprinter::IntegerAtom` class, with slightly different implementations for 1.8 and 1.9.