Add API to release the EventStreams to InputEvents.

Because Demo.py running under X imports the application, the
EventStream is not detroyed when the application exits.
Hence the input devices are not ungrabbed, and the mouse
and keybaord no longer work in X.
Merge branch 'integration' of https://github.com/tipam/pi3d into integration
Merge pull request #52 from rurwin/integration

Add API to release the EventStreams to InputEvents.