Update version to v0.7.13
Demonstrate Animation#preload method in example code
minor demo change

- reduce the number of layer options
  so that it all fits on smaller screens.
Promise.sequence: validate promiseFn arg
TileAnimation: Fix error thrown if map unset during preloading

- A layer cannot preload without a map. So when the
  masterLayer's map was unset, layer.preload threw and error.
  Fixed this by caching the masterLayer's map when preloading
  starts.
  Calling TileAnimation#preload with no map set to begin with will
  still throw an error (expected behavior).