Adds AmazonEchoControl plugin, Fixes OpenHAB plugin

[deinhofer]

This PR substitutes #325 as it contains several improvements.

• Adds plugin to connect to Amazon Alexa devices via OpenHAB using the AmazonEchoControlBinding
• Fixes OpenHAB plugin: The plugin used to XML for data exchange to the OpenHAB instance, which is not supported any more. Use JSON instead
  • added implementation of actionString property. So it's easily possible to send item states to OpenHAB also from AsTeRICS Grid.