Adds option to enable write operations to write to all servers in list

Adds an alternate New method, NewWithRedundancy, that will enable the redundancy option in the client. This caused the client to bypass any shard selection during write operations (set, add, del, incr/decr) and just write to all servers in the list given on create.

Refactored the tests a little to reduce code duplication when testing redundancy, should be easier to understand if you just look directly at the test file (rather than the diff).

@jduff @snormore can you take a look please, thanks!

/cc @tobi