Allow fields to be omitted from list/detail form of resources

This is a pull request based on the patch by @funkybob in #526. Most of the credit should go to him. Also addresses #18. Things I added to the patch:

* Tests for the patch
* ``use_in`` can be a callback that is passed a bundle.
* ``use_in`` defaults to ``all`` if it is not a callable, ``list``, or ``detail``