CYB-251 "Likes" not being scored

 * previous implementation depended on the record being created already existing
 * gets the poster's canvas ID from the Attachment or MediaUrl model instead.
 * in the spec, the after_create will overwrite the specified gallery_id so an update
    is done on that attribute
Merge pull request #146 from coyote/credit_being_liked

CYB-251 "Likes" not being scored