eddie-communication: implement communication layer APIs for higher layers

The communication API is composed of:

• connect: set up the local CoAP node and connect it to the network
• disconnect: tear down the local CoAP node
• discover: get the list of all available resources in the network
• send_message: send a message to the specified resource (ip, port and resource uri), using the correct CoAP method, specifying both the query and the body of the message.
• receive_message: return the message with the corresponding token value. This message is the answer to the previous request made by the CoAP node.