add support for multiple memcache servers and options to EpiSession

Added support to allow for multiple servers and set memcached options.

 $hosts = array( 'host1', 'host2' );
 $ports = array( 11211, 11212 );
 $options =  array( Memcached::OPT_DISTRIBUTION => Memcached::DISTRIBUTION_CONSISTENT,
                            Memcached::OPT_LIBKETAMA_COMPATIBLE => true );

 EpiSession::employ( EpiSession::MEMCACHED, $hosts, $ports, false, 86400, $options );

Also supports array of 1 -> N:

 EpiSession::employ( EpiSession::MEMCACHED, $hosts );