Extract client handling into Client module

[dzunk]

Closes #22

Extracted class and instance-level management of the Redis client object into a new Client module. The client still defaults to Redis.current, which can be overridden at the class-level for all time series, or provided as an option to the instance for only one series.

Moved argument parsing into the Client module. Args passed to the cmd call will be automatically flattened, compacted, and have Time objects converted to their millisecond timestamp representation. This cleans up a lot of repetitive code at the call sites, and also means that cmd can be called with positional arguments, eliminating an intermediate args array.

Note that the class-level methods like .query_index can only use the class-level client.

Removed the @retention and @uncompressed instance variables.

• They could be inaccurate if you instantiated a new object without calling .create
• Retention information is available in the .info hash
• See Add uncompressed param to TS.ADD, TS.INCRBY, TS.DECRBY #31 for pending changes to UNCOMPRESSED