Add gulp.js

gulp.js is a streaming build system of Node.js community.
With gulp.js, we can define tasks to help the project lint,
preprocess and test JavaScript or other front-end source files.
Add gulp-jshint task.

The task is to lint JavaScript sources including plugin files and
gulpfile itself.
Lint plugin files.

With gulp-jshint.
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.