BibUpload: new --callback-url CLI parameter

• Add new --callback-url CLI parameter to bibupload, in order to provide a URL to callback at the end of the execution of the bibupload task, to POST a JSON-serialized response containing the detailed outcome of the task execution. (See the BibUpload Admin documentation for the detail of the current structure of this answer).

• Add new callback_url form field to robotupload handler in batchuploader to allow external clients that use robotupload to profit from the corresponding new functionality added in bibupload. (closes #217)