Provide simple Scala API

[tdauth]

A REST API might be too slow when the state machine is updated constantly.
Hence, provide a simple Scala API which can be called directly in Java.

[beatngu13]

[…] Scala API which can be called directly in Java.

We still go over network, don't we? This sounds like RPC… 🙈

[tdauth]

[…] Scala API which can be called directly in Java.

We still go over network, don't we? This sounds like RPC… see_no_evil

This API should not be called via network. It would have to be used as direct maven/sbt dependency of the service which creates the state machine.

[tdauth]

Done, open more issues for specific features.