Added an 'upload_progress_jsonp_output' directive that sets templates that work with jsonp.
Make jsonp parameter name configurable with the 'upload_progress_jsonp_parameter' directive.

Signed-off-by: Brice Figureau <brice@daysofwonder.com>

 * Added a content handler that overrides the original handler (but calls it). It is
used to add our own read_event_handler.

 * Implemented a read_event_handler that calls the original handler, and store the rest/content_length in our
own node.

 * modified the different handlers to use the new node fields

This version contains:
 * new timeout to remember connections after they are finished to be able
 to report termination on this connection to upload progress probes

 * handling of HTTP error 413 (request entity too large)

 * First version