Add agent registration endpoints

This adds PUT /api/5/agent/bridge and PUT /api/5/agent/gateway endpoints for "menshen agents" running on bridges and gateways to make calls to in order to register themselves.

These endpoints are authenticated with a x-menshen-agent-auth header which should be the hex encoded HMAC of the body of the message.

When menshen receives a request to one of these endpoints, it will either add that "resource" (gateway or bridge) to its "registry" or if it determines that the resource already exists in the registry it will update that resource's "last seen" property.

This also adds an optional "last seen cutoff" option to menshen where menshen can choose whether it should show resources that haven't received an update within the last N time. If that option is set to zero then menshen won't take any last seen information into consideration when returning resources to clients.

Addresses #56 (closed) and is part of the work of menshen_agent#2