Add support for sbt-web 1.1

Add support for using upcoming sbt-web 1.1 with play 2.3.

Filter out asset directories from the reloader classpath so that dev reloading does not do a full reload on asset changes. In sbt-web 1.1 multi-module builds there are extra class directories on the classpath, with the assets in webjar format, and these are marked with an attribute.

Add a switch for disabling the asset aggregation built into the play plugin.