Integrate OSGi plugin mechanism on gateway

[danbim]

The mechanism should allow:

• to add/remove additional drivers (a.k.a. DeviceAdapter and DeviceAdapterFactory implementations) at runtime
• to add arbitrary plugins that use the GatewayEventBus and the currently connected devices.

If straight forward it could now also provide support for virtual testbed devices that can be configured and instantiated by the user. If not so easily realized leave this for another issue later.

[danbim]

The plugin mechanism is now fully working. Please see the MockDeviceAdapter implementation to learn about how to write an OSGi-based gateway plugin that can be installed and updated at runtime.

Plugins that want to provide a driver for unsupported device types must register their DeviceAdapterFactory implementation with the DeviceAdapterRegistry upon activation and deregister upon deactivation. TR core will always prefer plugin drivers over the built-in drivers, allowing users to upgrade the built-in drivers at runtime with recompilation.

The gateway configuration file shows how to configure the gateway to look in a certain directory for plugins.