Asset re-factorings

There's a new "assets" task such that "assets in Assets" will provide the "resourceManaged in Assets" folder nicely populated. Similarly "assets in TestAssets" will provide the contents of resourceManaged both in the Assets and the TestAssets scope i.e. all of the module's assets are present in the target/public-test folder. The copying will be efficient given the use of sbt's Sync functionality.

In addition WebJars are now automatically extracted into the target/public and target/public-test folders.

There is a second commit to this PR that extends the re-factorings further by having all source file plugins return a Seq[(File, String)] i.e. a sequence of relative paths of files to be landed in the target/public and target/public-test folders. The nice thing about this approach is that nothing gets copied until "assets" or "pipeline" are invoked, and then things are copied just the one time (unless they change between builds of course).