* 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.
