Make collection field of ReferenceField optional.

Currently, there is no way to represent the reference to itself.
This patch makes collection field optional. The reference field without
collection field will be treated as raw object id.
Run Play application in JavaScript console.

Many Play libraries depend on Play application. The same reason with
plugin test runner.
Add test case for ScriptableUUID.

It tests the basic functionality of the module.
Throw an exception when readFile can't find the file.

Currently Resouce.fromFile doesn't throw an exception when there's no
file for the provided path. According to the scala-io document, it
should do. I fixed the problem with check the file existance and throw
an exception manually.
Add GET request routing configuration to route plugin.
Add content type to response.

This patch uses plain/text as the default value of contentType, but the
play framework supports contentType inference. Fix this line to use this
feature.
Add statusCode to response.

Default status code is 200.
Modify response.js to use ResponseInternal directly.

The logic for the functionality was separated into a JavaScript and
Scala files. In this case, if we wanted to change the logic, we would
have to change both of them. This patch merged the logic in the
JavaScript file into the Scala one.

Also a test case for ScriptableResponse has been added. Some properties
have also been added to test the functionalities.