Switch to leaflet.js

[codemasher]

This will however require a custom leaflet build to implement the tile getter to speed up map loading.

• several plugins are needed and to be found.
• switch to the geojson verion of the objectives.json

[codemasher]

Custom tile getter: https://github.com/codemasher/Leaflet/commit/b2577e5a05b45f3f83f0bf8bdbe36389bff61628