Issue: Saving Checkbox Values

This commit fixes an issue where checkboxes could not be edited for inline edit or all field edits.

When a BooleanField is directly passed to the to_database method, you are not able to accurately check to see if old != new. To fix this, I added a new getChanges method to the BooleanField class that will first do a comparison on the raw boolean value (true, false) and then get the value needed for the database. I then made sure that we use that new function when editing all fields together.

Additionally, I added a getClean method to the BooleanField class so that $this->_clean for the field would be set to either true or NULL depending on if the box is checked or not and would accurately save to the database when calling $form->saveAnswers()