Extracted HttpErrorHandler interface from Global

* Play will by default try and load one of these from the root package
  called ErrorHandler
* If none is found, it will fallback to one that delegates to Global, so
  that Global.onError etc methods still work for legacy apps
* Added a new mechanism to the Reflect utils to load bindings for either
  a Scala interface, or a Java interface with an adapter to the Scala
  interface
* Updated documentation, including removing documentation on old Global
  methods
Merge pull request #3444 from jroper/http-error-handler

Extracted HttpErrorHandler interface from Global