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.
Load configuration from current path if there's no Play app.

There could be no Play app when it's for tests or console. At that time,
the same configuration still need to be provided to modules.

This patch changes BeyondConfiguration to find the configuration
from the current app path.
Fix encoding bug in ScriptableFileSystem.

The `string` and `write` methods of scalax.io's Resource implicitly take
a `Codec` instance, not a string.
Add test case for ScriptableFileSystem API.

In order to test the module, some assets are added.
Return undefined, if the field is optional and no values are set.

Currently, getting the value from optional field occurs error if value
is not set. It's a bug.

This patch makes it return an undefined.
Fix typo. ObjectID -> ObjectId.

The name had been changed after
cf55bf4e36d5c9138e126b6df4dea519dccd1771.