Improved documentation.
Updated documentation.
Gave BindingTarget __slots__.
BufferGroup and BusGroup no longer inherit from Sequence.
Bus now inherits from BindingTarget.
Implemented ControlBusGetContiguousRequest.response_specification.
Modernized names in ResponseManager.
BusGroup now inherits from BindingTarget.
Implemented BusGroup.fill().
Implemented BusGroup.get().
Added BindingInput and BindingOutput to bindingtools.

These are concrete subclasses of the now abstract BindingSource and
BindingTarget mixin classes, for testing purposes.
Cleaned up Binding integration in miditools.