WV-793: Fixed arctic proj4 definition which was preventing the demo from
working on Chrome.

Upgraded Leaflet to 0.7.2 and Proj4Leaflet to 0.7.0. Versioned library
directories.

OpenLayers 2 demo was incorrectly using proj4-2.0.0. Switched to using
proj4js-1.1.0.

Links to source now use the release branch.
Send headers to tell the client to always revalidate
Updated README with new library versions and proj4js correction