Get livereload.js locally

A copy of livereload.js is embedded in the repository. This version is used by the server.
It can be updated through the `bin/update-livereload` script.

The reason for not updating the `livereload.js` are:
* we do not want to possibly inject a bugged `livereload.js`
* we do not master whether or not the `livereload` directory is owned by the user that launch the server.