Get MSAA objects from Windows events; bugfixes

* Remove 'mstools' directory (unsure of redistribution terms)
* Remove duplicate 'yeild' in msaa.py
* Add method to get IAccessible Element() from Windows events
* Add keyboard-focus.py to act as companion to inspect, but tracking
  keyboard focus instead of mouse cursor.
Legal clarifications; spaces instead of tabs

* Added LICENCE to explain my understanding.
* Switched from tabs to spaces for keyboard-focus.py