Centralizing code to detect user activity in screen saver

 * Code now sits in a separate module easy to integrate to other screensavers.
 * Three functions now will help clients work a typical draw loop wih no delay.
 * Added robustness with initial load delay, fixes annoying typeahead false positives during SDLC.
Final changes and more testing on the PI

 * added missing cleanup
 * clarified documentation in source
 * added programmable timeout
Merge pull request #85 from KanoComputing/ssaver-refactoring

Centralizing code to detect user activity in screen saver