Continuation frame handling for WebSockets

Also implemented backpressure.
Merge pull request #2683 from jroper/websocket-text-continuations

Continuation frame handling for WebSockets
Rename templates/README to README.md
Activator props and tutorials for play-java and play-scala templates

* Added tutorial files and activator.properties
* Use .gitkeep files to preserve empty app/assets directories
* Add 'preview' to template names for milestone builds