Add optional authentication to carbon-agent protocol

Currently carbon-agent is unauthenticated, which is problematic for users who wish to run Graphite on a non-private network. A simple solution would be to add an optional configuration parameter for carbon-agent that is a pre-shared secret key. If this secret key is defined, carbon-agent could send a random string to any connecting client, who would then be required to reply with a hash of this string concatenated to the secret key. If clients fail to do this, they simply get disconnected. This is basic challenge-response authentication.