minor tidying
Merge pull request #51 from rurwin/integration

Implement InputEvents subsystem and update Silo.py to use it.
Now demos work whichever directory they are run from - fixes https://github.com/tipam/pi3d/issues/49
Now demos work whichever directory they are run from - fixes https://github.com/tipam/pi3d/issues/49
Now demos work whichever directory they are run from - fixes https://github.com/tipam/pi3d/issues/49
explain latest way of running demos
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.