add websocket directive that defers to an ActorRef

**Issue by [rkuhn](https://github.com/rkuhn)**
_Friday Jan 15, 2016 at 13:10 GMT_
_Originally opened as https://github.com/akka/akka/issues/19475_

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using #19474 this should place a rate limiter on the inbound messages and pipe to/from the Actor

Start by reading the referenced issue #19474 and the websocket directive implementation; the issue does not name specific files or tests. Establish how the directive should defer to an ActorRef, apply the inbound rate limiter, and pipe messages to and from the Actor, then define tests that demonstrate the complete behavior.

Written by the indexing model from the issue text.