ExecutionContexts for Java Futures

- Changed F.Promise methods to take an ExecutionContext
- Changed F.Promise ExecutionContext to use ActorSystem dispatcher
- Made legacy ExecutionContext available just in case
- Support for preserving context ClassLoader and Http.Context ThreadLocal
- Deprecated some play.libs.Akka methods
- Deprecated play.api.libs.concurrent.PlayPromise
Merge pull request #1102 from richdougherty/java-future

ExecutionContexts for Java Futures