Relative hyperlinks, so the plugin works when Etherpad is installed under a subfolder

For this to work, determining the absolute url needed to move from the server to
the client, i.e. from controllers/fileUpload.js to static/js/fileUpload.js
Merge pull request #13 from codingisacopingstrategy/master

Relative hyperlinks, so the plugin works when Etherpad is installed under a subfolder