Bugfix when there is no title tag on a page

Before this bugfix, the title would be set to the last one we preloaded.
Also, I already saw a page without title having `document.title` set to `false`, but I don’t remember the browser or the context.
Add `click` event (brings compatibility for NProgress)
Permits putting instantclick.js in the tests directory

Handy for testing an older version of InstantClick (to spot a regression) or for making it accessible on a webserver.
.gitignore