REST resources for bindings, things, discovery and inbox.

Added new REST resources: BindingResource, DiscoveryResource, InboxResource, ThingResource and ThingTypeResource.
Extended ItemResource and moved to org.eclipse.smarthome.io.rest.core bundle.
Extended DiscoveryServiceRegistry with methods for supported thing types and bindings.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>