helper.js should be the first loaded file

follow in-viewport methodology: put in helpers/
We must references assets using absolute paths

Paths cannot be relatives to the js tests, it was only a `feature` of
mocha-phantomjs.

Also, it breaks `npm test` for now, see metaskills/mocha-phantomjs#77
Merge branch 'master' of github.com:vvo/lazyload