Upgrade to Akka HTTP 0.9

Everything compiles and tests work so I'm happy for this to be merged into master.

However there is still work to do once the next version of Akka HTTP is released. As of Akka HTTP 0.9 the Akka HTTP API hasn't fully switched over to use Akka Streams. Once Akka HTTP makes the change to Akka Streams I'll update the Play backend again.

Also once the Akka Streams API settles down a bit I plan on providing direct conversions to and from iteratees and enumerators. At the moment the conversion goes Play enumerators/iteratees ↔ Reactive Streams publishers/subscribers ↔ Akka Streams sources/sinks, which is pretty inefficient and also means we're not really testing out the Akka Streams API properly.

Cc @sirthias, @jrudolph.