closes issue gh-278

This needs closer review. 
First, as per conversation with Amy Kirschner, this implementation actually in the skillbank links to OFFERS ONLY. This was my intuitive understanding of skillbank (skills offered). Amy agreed and this is what has been implemented.

The change in the category_controller should be reviewed, as there are json and xml formatting segments of which I don't know the semantics and thus just inferred a correct response.