Major changes. May break stuff.

1. Added source option. Inspired by jQuery UI Autocomplete. It works
just like the one there.
2. Added onChange event that's triggered when any dropdown in the group
changes value. It also passes the values from all dropdowns in that
group as an object.
3. Using dropdown name attribute as fallback to paramName, if it's not
defined. If nothing is defined then the dropdown value will not be
passed to any relying functions.
4. textKey and valueKey is now obselete. Use the source option to define
dropdown items.
5. Parent options are no longer passed to dropdowns. This is no longer
necessary.