Allow modules to change the redirect destination and message

Currently invite_accept() hard codes various redirect paths and status messages to be displayed when a user accepts an invite. There is no way for a module to change these. This patch adds a call to drupal_alter() before the end of the function.

Example implementation:

/**
 * Implements hook_invite_accept_alter().
 */
function MODULE_invite_accept_alter($data) {

  $invite = $data['invite'];

  if (!empty($invite->data['gid']) && INVITE_VALID === $invite->status()) {

    $data['redirect'] = 'node/' . $invite->data['gid'];
    $data['message'] = t('Please login or register for an account to begin working on this item.');
  }
}