Support named layers for MapBox, drop deprecated layers.

The mapbox.mapbox-streets and other endpoints are no longer available,
but users can make free maps in accounts and use the map ids. This
removes the deprecated endpoints and adds the ability to add layers as
MapBox.foo.bar as an upgrade path.
Update leaflet-providers-0.0.2.js

Add Esri's Wold Canvas - Light Gray Base
http://server.arcgisonline.com/ArcGIS/rest/services/Canvas/World_Light_Gray_Base/MapServer
Update demo.html

Add Esri's gray canvas to basemap demo
Update leaflet-providers-0.0.2.js

Add more Esri basemap options
Update leaflet-providers-0.0.2.js

Add WorldStreetMap options for full attribution
Update demo.html

Add new basemap options to demo
Update leaflet-providers-0.0.2.js

Remove option for NatGeoTopo
Update demo.html

Remove option for NatGeoTopo in demo
Merge remote-tracking branch 'geo_steve/master'

Merge in changes from GeoSteve/leaflet-providers to add more ESRI.
Merge remote-tracking branch 'tmcw/mapbox-layers'

Get mapbox updates from tmcw/leaflet-providers /mapbox-layers