Dynamically set the slides gap to the window height.

This stops slide intrusion. Also add the window height as a
bottom margin on the slides container to stop the last slide
from being on the bottom.

Cache the window jQuery object since we use it multiple times.
Fix H1 slide content vertical centering for any viewport size.
Merge branch 'viewport-size-aware' into add-to-gist-io
Update script with new goodies from updated gistdeck.js