Change Scalastyle to work with Travis CI.

Scalastyle was intended to work with `sbt test`, but it didn't work.
This commit removes the lines in build.sbt for it and just adds the
command in the Travis CI configuration file.
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 ScriptableHttpClient.

ScriptableHttpClient is the module to send HTTP request from the Beyond
plugin. It refers to Node's SuperAgent package for its API design.

The test case is added as plugins/test/http-client.js
Fix the bug that reference field is not working.

I changed to use ScriptableObjectId instead of using ObjectId itself for
reference field. But I missed to fix here.