introduce System, Pocket::system, and tests. add section headings for groups of functions.
pocket.systemForEach and System.forEach and tests
update readme about systems
full workflow gulp file

clean, lint, html, sass, src, demos, test, browser-sync, watch
(http://vincent.is/introducing-people-to-gulp/)
begin bounce demo

context-2d component
complete basic bounce demo!

a single ball moving back and forth across the screen
pocket.tick destroys keys and runs all systems

basic building blocks for a game loop!