Initial suggested PSTbox  related changes

PSTBox
------------

- removed items from _newDataTypes that already exist in parent
- Added an instance attribute that was missing from __slots__

PSTBoxByteChangeEvent
-----------------------------------

- removed items from _newDataTypes that already exist in parent
Updated new event type

Is new name OK? 

Let me know about whether the new event really needs prev_byte and current_byte fields or not.
Added new event type constants
Added PSTBOX_BUTTON event table creation
Updated comments.++

Change new event class name in PSTBox.EVENT_CLASS_NAMES
Merge pull request #1 from isolver/patch-1

Initial suggested PSTbox  related changes