Refactor socket code to array
Add multiple server support to memc:connect()
Consistent hashing with ketama - first cut

This adds consistent hashing using the ketama algorithm. The algorithm
implementation is that from the memcached C client. A couple of notes:

- The ketama test requires a second memcached server be running on port
  11212
- Requires "bit" and "ffi" modules which are part of Luajit