Integrate unit testing framework with sbt test discovery.

- Tests can now be run using the test facilities built in sbt. To use it
  you need to add a dependency on a Scala.js test framework (for example
  Scala.js jasmine test framework) and add a marker interface
  (`scala.scalajs.test.Test`) to your test classes (or extend
  JasmineTest when using the Scala.js jasmine test framework).
- The default Scala.js test framework and Scala.js test bridge class can
  be changed using sbt settings.
- A reference implementation of the test framework using jasmine is
  provided.
- Added experimental support for `env.js` in the run and test
  environment. Just add `env.js` webjar as a dependency to your project.
- Run command now searches for objects that have a `main` method (normal
  sbt behavior).