Implement Parallax's register-image API call

Implement Parallax's register-image API call. At a minimum, the register-image call must accept image metadata and store that metadata in the registry data store. The metadata that can be stored is, at least:

* image name
* public image?
* image type ('kernel', 'ramdisk', 'machine', 'raw') any more?
* state ('pending','available','disabled') any more?
* architecture (list arches...)
* creating_user
* project_id (can be null)
* created_at
* updated_at
* deleted_at
* deleted?

In addition to the above standard metadata fields, there should be the ability to add custom metadata key/value pairs for an image. Kind of like a simple tagging taxonomy system.