Store false/unchecked value of attribute as well

The value was only stored when the field key was present in $_POST, i.e.
when the checkbox was checked. This caused the attribute value to be 'locked'
as true if it was ever stored as true/checked.
Inconsequential syntax change
Merge pull request #34 from jjmmkk/master

Store false/unchecked value of attribute as well
Update README.rst
Fix JS error if the title contains single quote
Merge pull request #37 from SylvainGuittard/master

Fix JS error if the title contains single quote
Converted README file to markdown
Added composer.json
Changed project URL to Github