Migrate to Leaflet. Looks the same, but takes more code.

The code is confusing, too.

One benefit: we don't actually add the HTML for a tweet or Instagram
post to the page until after the user clicks. That saves a bunch of
image loading.
Fix relativeXyToLatLng() and implement clustering
Merge branch 'leaflet'. Breaks marker images.

Conflicts:
	css/style.css
	js/main.js