Update Leaflet.js to 0.5.1

Downloaded from https://github.com/Leaflet/Leaflet/zipball/v0.5.1
Changelog: https://github.com/Leaflet/Leaflet/blob/master/CHANGELOG.md#051-february-6-2013
Merge pull request #941 from redapple/leaflet-0.5.1

Update Leaflet.js to 0.5.1