Fix disable support for Opera

Changed the logic for determining if a checkbox should be disabled to start with to use the 'is' jQuery method to the 'prop' jQuery method as not all versions of Opera support the :disabled pseudo selector.
Fix disable Opera support

This change will fix support for disabled in the most recent version of Opera and several older Opera version.
Merge pull request #266 from nephiw/patch-3

Fix disable support for Opera
Merge pull request #267 from nephiw/patch-4

Fix disable Opera support