Add grid experiment

Hi Tom, I've enjoyed tinkering with your animation library the last few years.

This branch adds a grid example.  Each grid cell is a sprite which can be animated, given children, made interactive, and so on.  I've been using this to create a boardgame with a hexagon map and a calendar grid with squares.  

There's also some minor spelling fixes and stuff.  I put the drawing code for making hexagons and triangles in the main library, hope that's ok.