Use gulp.watch() in conjunction with BrowserSync's reload() method

instead of BrowserSync's 'files' option.
This seems to be the recommended practice, see
http://www.browsersync.io/docs/gulp/ and
https://github.com/shakyShane/browser-sync/issues/154
And it also seems to notice the changes much faster.

Added main.js so people can see how changing a JavaScript file also results
in the browser getting automtically reloaded.