Added layers attr, multuple baselayers and watch
Updated example with add buttons
Fixed bug activatinc layer when watching
Merge pull request #109 from JaumeFigueras/layers

Multiple Layers (baselayer XYZ)
Changed to the Leaflet CDN
Merge branch 'multilayers'. Implemented by @JaumeFigueras:

https://github.com/tombatossals/angular-leaflet-directive/pull/109