Methods for checking that all assets have been specified

Add a method to Section model that checks that all containers in
the layout have an asset.  Add a method to the Story model that
iterates over the story's sections and check that each section has
assets for its containers.

Closes #941
Explicit top position for header.

Addresses #942
Only toggle sticky header on initial render, window resize

It's a waste to check the header percentage and toggle the header's
stickiness on every scroll event.  Just do this on the initial
render and when the window is resized.

Addresses #942
Update readme for #942