Enabled (backwards compatable) server callbacks using JSON-RPC 2.0

Since in our system we use plupload to handle uploads and access the files through an ID that the server creates in our database we needed the server callback functionality.

Some features:
Error messages can be returned from the server even if the HTTP request was successful.
It allows for return values from the server when the upload was successful.
It creates a new event called ServerFileIdReport. (Change to a more generic name if you wish, it can be used for any JSON serializable return value.)
Tested and implemented in the HTML5, Flash and HTML4 runtimes. Others can be implemented with ease about 4-5 lines of code. Only reason I didn't implement them is because I had no environment to test it and our system does not require other runtimes in the first place.