Add apigility integration

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Provide a generic messagebox endpoint to receive commands from the client. Each command type should have an own messagebox, so that you can document the different commands like you would document resources.

A messagebox should answer with a HTTP Status code and with a hypermedia link to an endpoint where client can check the status of the command: pending, succeeded, failed, ....

Provide helper to add hal links to resources pointing to available commands for the resource.

Also provide an event-stream endpoint, so that a client can query for published events.

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This is covered with prooph/no-mvc