Show area When Polygon

I had to:
* fix a broken dependency when installing
* include a mapbox.js plugin on the header
* edit an unrelated section of the code to validate `jshint`.
* Add `jshint` to `package.json` for travis.


 ![geojson_io](https://cloud.githubusercontent.com/assets/434029/3207622/f030963e-edf0-11e3-968a-f4a5afc97cd3.png)
We should use leaflet-geodesy from npm rather than from the CDN, to reduce request load and manage versions