Changed the index to be >= 0 to add offset/push/pull-0

- Changed the index to be greater than or equal to zero so that offset-0, push-0, and pull-0 get added to the css file for html usage as "col-md-offset-0" to reset the offset/push/pull if you want to go from something that isn't zero, back to zero. This was changed from 3.0.0 apparently, as with 3.0.1 you can't uses these classes in HTML. You can generate the same effect on a class through less mixins like ".make-sm-column-offset(0)", but if using just bootstrap.css there was no way to reset them back to 0.