allow JSONService to return request objects

JSONService.sendRequest and JSONService.sendNotify used to return a request ID
and an integer, respectively.
This patch adds a `return_object' flag to the JSONService, ServiceProxy and
JSONProxy constructors.  When ̀`return_object' is True, JSONService.sendRequest
and JSONService.sendNotify will append the request object to their return
value.
This allows to access the JavaScript XMLHttpRequest object, on which the abort() method can be called in order to interrupt a very long request.

Resolves #796 

Should be backward compatible with existing code.