Code cleanup

I just did a quick pass through the .module file and cleaned the code a bit.

• reformat code according to drupal code style
• changed and added some comments
• array_unshift instead of array_merge in the hook_element_info_alter
• fixed a little bug: not required elements with value 0 shouldn't be treated as 'empty'
• changed form_set_error to form_error

todo:

• add comments to rule/filter callbacks instead of a single one above the whole block
• perhaps use filter_var instead of custom checks/regexes/etc