nsqd: adding user agent string to client

Adding user_agent string for client `IDENTIFY`. refs #189. This is now exposed to nsqadmin.

Where was the plan for this to go in nsqadmin?  The channel screen is already pretty busy with the addition of client attributes.  Even though there is a *node* screen for NSQd nodes, there is no *client* screen where you can break down what an individual client is doing. Thoughts on that? Anything else in mind?