Support multiple upload types, multiple atoms creation

If the atom type does not use multiple upload, there is nothing to change.

Since Scald 1.1 multiple atoms can be created at the same time. To simplify the API, one function argument has been changed.

Scald 1.0:

hook_scald_add_form_fill(&$atom, $form, $form_state) {
  // do stuff with $atom
}

needs to be changed to work with both 1.0 and 1.1+ (the easiest way, but it does not support multiple atom creation):

hook_scald_add_form_fill(&$atoms, $form, $form_state) {
  $atom = is_array($atoms) ? reset($atoms) : $atoms;
  // do stuff with $atom
}

In order to keep an atom provider module compatible with all Scald versions, the first argument should be checked to see if it is an array.

To use the new feature (plupload integration), module should implement hook_scald_add_atom_count() to check the $form_state value and decide how many atoms will be created.

For more information, see the issue and read scald.api.php file for an example.