implemented doubleClickZoom

Hi,

In angular-leaflet-directive/src i expanded the defaults objects, and added support for doubleClickZoom in the same fashion as maxZoom and minZoom attributes are supported. 

Please discard commits to the other files, consider only those few lines in src/angular-leaflet-directive.js.

For the future would be cool to reflect the options object of leaflet map() so that  any parameter can in the local scope.