Made Global handlers return futures

The results refactoring ended with Global handlers, such as onError,
onHandlerNotFound etc, returning SimpleResult, which meant they couldn't
be asynchronous.  This changes the type to Future[SimpleResult].

Also extracted code samples out of Java Global docs, and updated all
docs for this.
Merge pull request #1545 from jroper/global-handler-futures

Made Global handlers return futures