Add pre canned RxError handler.

Goes someway to addressing #306, but hopefully we can find a way to make this unnecessary.
Don't link to Rx Javadoc until they get it current.
Reenable Rx javadoc links.
Some doc tidy up.
pac4j: Make user profile available in request registry both as concrete type and `UserProfile` (#307)
Use Rx plugins to provide a default error handling strategy.

Closes #306.
Rename SuccessOrErrorPromise to Promise.
Javadoc fix.
Install Rx support for site app.
Fix use of Rx in tests.
Javadoc fix.
Codenarc fix.
Some fixes for Rx error handling.
Fix after GitHub changed their issue data.
More fixes for Rx error handling.
Make it possible to the Rx integration for multiple Ratpack apps in the same JVM.
Make the unit tests run on an event loop thread so the infrastructure is properly initialized.
Some tidy up of the exec controller thread binding.
Start of a chapter on async.
Better client side caching for the site.