Remove dependency on feeds nodes

Issue Summary

Currently this module is fully dependent on users using feeds via feeds nodes.

It doesn't allow for users using the feeds import forms (the ones at /import/[importer_machine_name]).

This means if each feed importer is going to be used only once you still have to have content type/node for feeds, which is way unnecessary.

For this change we need to:
* Change the schema so that you aren't forced to have an nid, and add another field for feeds importer id and make the primary key a combination of the two. OR there could be two tables.
* Add the feeds_imagegrabber_form() to the feeds formatter settings area somewhere, so imagegrabber can be set up for each feeds importer (then they can be overridden at the feeds node level if desired).
* Change the logic in feeds_imagegrabber_feeds_set_target() to allow for the new feeds importer level settings.

Then the feeds importer pages (the ones at /import/[importer_machine_name]) can use these options.

There is a similar issue for this for drupal 6 at #1063564: Add imagegrabber to feeds-import-form (standalone form)

This is potentially a bug also but it is a major change so making it a new feature.