Expose the storeblob service in API

In case it is helpful, this is python-launchpad-bugs' implementation of a convenient storeblob API. We have used that in apport for years:

        import storeblob

        def progress_callback(sent, total):
             print 'uploaded %i of %i bytes' % (sent, total)

        use_staging = True
        blob = open('/tmp/data')
        ticket = storeblob.upload(blob, progress_callback, staging=use_staging)

This is the multipartpost_handler, as required for the storeblob API.

http://pypi.python.org/pypi/poster/ implements the core already, we could trivially expose that as the launchpadlib interface : if there is interest in taking this route I can package it up pretty quickly.