Authentication & Authorization

[mcollina]

The authentication support is twofold:

• provide a callback to accept or reject a client.
• add a simple json-based backend for it.

Authorization support is manyfold:

• add a callback to authorize a publish.
• add a callback to authorize a subscribe.
• add a simple json-base backend to specify them.
• modify the CLI to add and remove users.
• modify the CLI to load and install the authorization system.
• modify the CLI to reload the JSON file when receiving an UNIX signal.
• Update the README for the handling of the users settings.
• Add the topics authorizations handling in the CLI.
• Formatting the credentials JSON.