Fix #301. Make docs play nice with https

@scw & @patrickarlt: Can try it now using `HTTPS Everywhere` and let me know if I missed out anything.

Few notes:
- I've changed examples to use `//` where appropriate but I didn't touch api-examples. I think those should stay with `http://` since they are for demonstration purposes only.
- `cdn.leafletjs.com` is `http://` only. We can use `jsdelivr.com` instead but it's not the latest version of leaflet so I'm against using it (for now).
- Two examples are using hardcoded MapService urls (http://raster.nationalmap.gov/arcgis/rest/ and http://imagery.oregonexplorer.info/) since they don't support `https://` yet. In the future this might change for one of them (raster.nationalmap.gov).