ZSet commands added

Almost there :dart: 

Hope to finish the API before dart 1.0 hits: https://groups.google.com/a/dartlang.org/forum/#!topic/misc/chOwECXphTw

Also have a look at how cool named parameters make this function call:

Future<dynamic> zrangebyscore(String setId, { num min, num max, bool minExclusive: false, bool maxExclusive: false, bool withScores: false, int skip, int take});

A lot of options and I think using named parameters is appropriate since most of them are optional.