Enforce HTTP error handling, add metrics and revamp timeout handling
Merge branch 'master' into clement-fix-timeouts

Conflicts:
	src/main/scala/com/wajam/nlb/client/ClientActor.scala
	src/main/scala/com/wajam/nlb/client/SprayConnectionPool.scala
	src/main/scala/com/wajam/nlb/forwarder/ForwarderActor.scala
	src/test/scala/com/wajam/nlb/client/TestClientActor.scala
	src/test/scala/com/wajam/nlb/client/TestSprayConnectionPool.scala
Changes after review
Merge pull request #21 from wajam/clement-fix-timeouts

Enforce HTTP error handling, add metrics and revamp timeout handling