Fix sep and delimiter properties in ScriptablePath.

JavaScript function's static properties can't be defined with @JSGetter.
We should define the properties in the `finishInit` function.
Add test case for ScriptablePath API.

It can be tested with `activator plugin-test`.
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.