Disallow add_creds with != 1 factor.

Since we don't have a way to signal which factors were added and
which were not, it seems better to just not accept more than one
factor per request until this has been thought through.
Better key handle parsing.

Parse key handles with pyhsm.util.key_handle_to_int to allow for
hexadecimal key handles.
typo
Fix a syntax warning.
Stop using cherrypy.log.

In order to get logs to syslog, let's just use our onw VCCSLogger
instance instead.