Add authentication support

* Wire support for AUTH command
* NSQDConnection will use AUTH after TLS, Snappy, Deflate negotiation and before SUBSCRIBE / PUB
* Updated README
* Refactored the constructor signature for NSQDConnection to take an options dictionary
How about
```coffee
if params.authSecret? and not _.isString params.authSecret
```
since I know you like to avoid using `unless`.