New projects default to Scala 2.11.0; template fixes

The main change here is to change Play so that by default new projects are created with Scala 2.11. I needed to introduce a new `PlayVersion` setting, because the version we want to use is different to the version that the Play sbt plugin is built with.

This PR also includes a few other misc fixes to the templates. The Twirl fix is actually a compile problem. This compile probably would have been picked up if `runtests` covered the template tests. We should make `runtests` run the template tests. (I'll look at doing that in another PR.)