Allow multiple score/member pairs to be inserted with zadd.

Functionality added in a backwards-compatible way without method overloading. A `zAdd` call would look like

    client.zAdd(key, score1, member1, (score2, member2), ...)

The multiple element version of zadd only works with redis >= 2.4.

Any tips on how to run the integration tests would be appreciated. `NaggatiSpec` is the only file that runs with `sbt`.