get_multi: Support an arbitrary collection of keys, not just a list
get_multi: Support an arbitrary collection of keys, not just a list

This is required for compatibility with Django.  Also fix an
IndexError if keys is empty.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

jaysonsantos/python-binary-memcached,jaysonsantos/python-binary-memcached