Log Message:
1.58 release:
*  Fixing a performance regression in v1.54 in Python 2, using cPickle again.
*  Support for "stats slabs".
*  get_stats encoding fix.
*  Pin Six version to >= 1.4
*  setup.py build process pulls version from memcached.py
*  delete() and delete_multi() now default the "time" argument to None,
   since the protocol doesn't allow a time in some implementations.