Fix #301. Make docs play nice with https

I've closed #319 since rebasing was a mess I didn't want to deal with :). I've revisited https docs issue and updated docs accordingly. Also, from now on we are supporting image service over https (https://raster.nationalmap.gov/ArcGis/rest).

/cc @scw @patrickarlt @tomwayson @jgravois 

See bellow for updated status from PR #319 

* 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://sampleserver3.arcgisonline.com and http://imagery.oregonexplorer.info/) since they don't support https:// yet.