[#1977] Implement a redeemablePromise
Update Eclipse debug documentation

As discussed in https://groups.google.com/forum/#!topic/play-framework/O2aoDOgY8Ac
Merge pull request #2034 from fernandoacorreia/ide-debug-doc

Update Eclipse debug documentation
Merge pull request #1999 from baloo/improvements/1977-Promise.empty

Added RedeemablePromise to the Java API, which is a Promise object that can be redeemed later with a value or exception. This gives Play's Java API an equivalent to Java 8's CompletableFuture or Scala's Promise.
Merge remote-tracking branch: upstream/master