Fix of innerText for FireFox

The innerText property is not in the standard and not supported by FF.
Therefore, it wasn't possible to work with the lessons in FireFox.
See
http://stackoverflow.com/questions/1836942/difference-between-innertext-and-html
for details.
Merge remote-tracking branch 'origin/ff-fix' into gh-pages

Conflicts:
	assets/app/main.js