Refactor query building and signatures.

- Enable autoloading in poll.php.
- Query parameters and hashing are now handled in a RequestParams object.
- Merge poll_all.php into poll.php.
- Use parameter hashing in poll.php too.
- FIX: $addressfield must be used in the queries too.
- FIX: all supporters were only displayed if the supporters had an entry
  in field_address.
Move backend code into it's own class.
Make the backend responsible for getting the right translation.
Plugin loader and selection in the admin interface.

- Unify how settings are handled.
Implement the webform-only backend.
Merge branch 'recent_supporters_backends' into 7.x-1.x