nsqd authorization

This is an attempt at creating an authorization protocol as originaly requested in #224

Specification:

nsqd TCP protocol will add a AUTH command with a json body that MUST be called before SUB or PUB when nsqd has authorization enabled.

Nsqd will make authorization requests against any auth server configured until it gets a valid response. It will pass along the metadata from the client, and the clients remote address. The auth server will reply with a list of topic/channels that the connection is valid for. The topic and channel names returned will be interpreted as a regex which will allow for the auth server to give blanket auth response in the form of `topic=.*, channels=[.*]` response and will specify a TTL for how long the authorization is good for. The auth request will be cached and re-tried upon expiration of the authorization metadata. (this will ensure that when authorization is removed from a auth server, you can ensure that it no longer applies after a specified ttl).

Authorization information will be stored on the connection for the life of the connection or until the TTL expires, and will be checked at SUB or PUB time.

It is expected that when using an authorization server, the nsqd HTTP endpoints are not exposed to untrusted clients. 

A simple auth server will be included that gives a UI to manage authorizations based on login, tls, remote_ip and optionally an oauth2 endpoint used to lookup login values. It is expected that other auth servers will be written for different authorization backends (like LDAP, ssl certificate checking, etc). Connections between nsqd and the authorization server are expected to be over a trusted network.

cc: @mreiferson
We're doing this synchronously in the `PUB` / `MPUB` path and let's say `authd` is slow to respond.  Since it's also going to periodically expire and re-query, what do you think about putting that responsibility in a goroutine that doesn't (potentially) affect the publishing of messages?

(it doesn't matter for `SUB`)