Gulp Build: Use Gulp to build project.

I've created the `gulpfile.js` build file porting the functionality from `Makefile` as is proposed in #702.

There are still a couple of tasks that are using the original shell command from `Makefile`, but they can slowly be improved.

To build the project we've to issue the following commands.

```bash
$ npm install
$ gulp
```

Also, I've added a `watch` task to rebuild app files when they change.

```bash
$ gulp watch
```