Add broker feature

[LukasPoque]

The S3I-Broker and the corresponding S3I-B-Protocol are basic features of the S3I communication.

This PR adds:

• S3I-B-Messages classes (Message, UserMessage, ServiceMessage [ServiceRequest, ServiceReply], AttributeValueMessage [GetValueRequest, GetValueReply])
• Basic interfaces for the Broker communication (BrokerInterface, ActiveBrokerInterface, PassiveBrokerInterface):
  • ActiveBrokerInterface for interfaces that inform you whenever a new message is available
  • PassiveBrokerInterface for interfaces where you need to explicitly ask if there are new messages
• An implementation of the ActiveBrokerInterface using the AMQP protocol (not usable for web)
• An implementation of the ActiveBrokerInterface using the REST API of the broker