Writer implementation

* Update README with Writer example and changelog
* Fixed nsqdconnection member initialization
* Slight refactor of nsqdconnection / ConnectionState to support writer
* Renamed NSQDConnection's SUBSCRIBED event to READY so that it makes
sense in both the Reader and Writer cases.
* Added NSQDConnectionWriter
* Additional check for Reader options parameters
* Added `lookupd.coffee` to refactor the logic for querying lookupds
for nodes.
* Updated `Reader` to use the `lookupd` module functionality.