Add HTTP notifications of admin actions to nsqadmin

This commit creates a global notifications channel on which the new
AdminAction type is sent, and a goroutine that reads the actions and
sends them as json blobs to the endpoint specified on the cmd line.

Deferring work on an audit page or some such thing to a later date
Merge pull request #117 from danielhfrank/117_post_admin_actions

post admin actions to remote http endpoint