Implemented WriterTopic
Merge branch 'refs/heads/topic/stateTopic' into spinoco/master

Conflicts:
	src/main/scala/scalaz/stream/actor/TopicActor.scala
	src/main/scala/scalaz/stream/actor/message.scala
	src/main/scala/scalaz/stream/async/mutable/Topic.scala
	src/main/scala/scalaz/stream/async/package.scala
	src/test/scala/scalaz/stream/AsyncTopicSpec.scala