Add task to test plugin JavaScript APIs.

Implement a sbt task to test plugin JavaScript APIs. The test framework
is written in JavaScript and placed in `plugins/test/lib`. The task
finds test cases in the `plugins/test` directory. By bdd.js, the test
cases are parsed as a task tree and then launched. The result is
reported by TestReporter written in Scala.

An example test case, `example.js`, is added in `plugins/test` to show
how a test case should be implemented.
Add ScriptableHttp.

ScriptableHttp is the module to send HTTP request from the Beyond
plugin. Basically, it's the thin wrapper for Play's WS API.