Made routes compiler its own plugin

* Improved the structure of routes compilation, so that it's more configurable
* Made the incremental compilation good
* Took many of the old integration tests for the router and turned it into a scripted test
* Wrote scripted tests for incremental compilation
* Included mocha tests for the JavaScript reverse router in incremental compilation

Do not merge yet - it depends on this:

https://github.com/sbt/sbt-web/pull/60