Add webjarsAt and publicAt url generation methods for Freemarker
Add webjarsAt and publicAt url generation methods for Groovy
Add webjarsAt and publicAt url generation methods for Jade
Add webjarsAt and publicAt url generation methods for Pebble
Add webjarsAt and publicAt url generation methods for Trimou
Only allow one route registration for each resource handler type

For example, you can only register once route/instance of a WebjarsResourceHandler.
This same limitation applies to the PublicResourceHandler, which I suppose could have
multiple served source paths.  If this is the case, subclass and register the subclass
instance.
Merge pull request #29 from decebals/resourceurls

Add webjarsAt and publicAt methods/helpers/extensions to all template engines