Inconsistent consistent hashing algorithm, not matching libmemcached

The version of hashring used by node-memcached does not correctly implement consistent hashing like libketama, which prevents using it to cache data between several different libmemcached clients. Updating to hashring 1.0.8 appears to have fixed this.

I have also updated the readme for new behaviour in hashring 1.0.8. I've also added a note about compatibility with other libmemcached clients using consistent hashing. Had to change the default algorithm to md5 as the crc32 implementation causes errors now.

I've updated all the deprecation warnings from the update and the tests show everything working fine.