Log a message when watchify finishes a build.

There was some gulp output when the original 'watch' task completed, but there wasn't any output when watchify rebuilt changed files. I assumed it wasn't working properly because if you run watchify by itself on the command line it writes a message every time it builds.

I also added some comments to answer some questions I had about how gulp, watchify, browserify, and vinyl work together in this example.
This also sets the name of the bundled output. I believe you're going to be saving bundle.js to dist/.