Add server JSON representation to drag events

I believe something like this would help web applications to better interact with users' Zotero libraries. These sorts of interactions are already possible through using the server API, but I think it would be useful for both users and developers to allow them without requiring the server as an intermediary.

This would enable:
  * more sophisticated interactions via drag-and-drop to browsers
  * use of Zotero data in offline applications
  * use of Zotero data not synced to the server for whatever reason
  * use of Zotero data synced to servers other than the one located at api.zotero.org without needing to manage multiple API keys/authorizations