Image api fixes

This fixes a couple of problems with getting and setting images over the API. Firstly it allows multiple images to be added to a document by sending an images array. It also includes an image attribute so that we're a bit more in line with the popolo spec.

Secondly it handles imaged uploaded to the popit server using the front end better in that it includes a url for them and also correctly handles the _id attribute of 'internal' images.
Does this `image` -> `images` processing also need to happen for PUT requests?