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.rhino.1.2.js`
  to the `resource` directory of your project.
- Run command now searches for objects that have a `main` method (normal
sbt behaviour)