Use local version of gulp in build steps.

Previously developers had to install gulp globally on their machine
to build the project. These changes changes that by using the already
locally installed version of gulp, removing the environment requirements
aswell as ensuring everyone are using the same version of gulp.

It fixes the VS Code build tasks, aswell as creating a npm script to trigger the watch task:

```bash
$ npm run watch
```

.vscode/tasks.json

@@ -1,6 +1,6 @@
 {
 	"version": "0.1.0",
-	"command": "gulp",
+	"command": "./node_modules/.bin/gulp",
 	"isShellCommand": true,
 	"tasks": [
 		{