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 assert.async for plugin test.

assert.async is a shorthand for the try-catch cluase of
asynchronous tests.
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.
Add plugin testcase for ScriptableHttp.

It tests basic get, post and put requests.