* mono/BusDriver.cs: Added.  This is a class for interacting with
	    the org.freedesktop.DBus service.

	    * mono/Message.cs: Added a mechanism to expose the message that is
	    currently being dispatched via the static Message.Current
	    property.  Added Message.Sender and Message.Destination
	    properties.

	    * mono/Handler.cs: Expose the dispatched message via
	    Message.Current when handling method calls.

	    * mono/Service.cs: Expose the dispatched message via
	    Message.Current when handling signal emissions.

	    * mono/Connection.cs: Bind dbus_bus_get_base_service via the
	    Connection.BaseService property.