Improved version of Rhino environment. Intergrates better with sbt for
`test` and `run` tasks.

- Tests can now be run using the built sbt in test facilities. 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
- Provided a reference implementation of the test framework using 
  jasmine
- Added experimental support for `env.js` in the run and test 
  environement. Just add `env.js` webjar as a dependency your project.
- Run command now searches for objects that have a `main` method (normal
  sbt behaviour)