Replace scalalogging with log4s. #65
slf4s was something else, this is log4s.
Merge branch 'develop' into topic/log4s

Conflicts:
	servlet/src/main/scala/org/http4s/servlet/Http4sServlet.scala
Bump to log4s-1.1.2.
Bump to Scala 2.11.4.
The master branch is 'master' again.
Merge branch 'topic/log4s'
Merge branch 'master' into topic/async-servlet-writer-atomic
Jetty needs an extra isReady check before flushing.
Fixes Tomcat in unchunked encoding.

Tomcat doesn't fire the onWritePossible callback before the first isReady
fails.