Allow specific checkboxes to override the global push_checkboxes

My use case is that you've set the `push_checkboxes` config option to true, but there is a single checkbox instances you want to **not** push checkboxes.  This change makes it so a checkbox will push a hidden field only if the global config is set AND the checkbox hasn't been explicitly set to not push.
Fixing issue revealed by Travis test
Adding a test for overriding globally pushed checkboxes
Merge pull request #286 from weotch/patch-1

Allow specific checkboxes to override the global push_checkboxes