Added the ability to use the web jars CDN.

WebJars now have a CDN -- see http://www.jamesward.com/2014/03/20/webjars-now-on-the-jsdelivr-cdn.

Added a configuration option to IWebjarsSettings: useCdnResources(). 

Added two static methods to WicketWebjars for creating Webjars JS and CSS ResourceReferences, based on the value that useCdnResources() returns.  If useCdnResources() is set, use a UrlResourceReference pointing to the WebJars CDN; otherwise, use the appropriate Webjars*ResourceReference.

Rather than using Webjars*ResourceReference directly, you'd use the static methods on the WicketWebjars  class to create them.