Added addCheckBox() function

Created new file named CheckBox.php under Section folder. Added
addCheckBox() function to Section.php and \Table\Cell.php files and
_writeCheckbox(..) function to Base.php file.
Test folder updated

Test folder updated for new file CheckBoxText
Fix deprecated method, unused parts, initial definition, and @method annotation
#156 Merge branch 'master' of https://github.com/ozilion/PHPWord