wordlist, fixed sublevels value retrieving from json

The key was searched as 'sublevel' but all our json files
use 'sublevels'.
numbers-odd-even, minor, added section
gletter, added support for using images

Now it is possible to develop an activity that reuses gletters
but that uses falling images instead of falling text.

To do some you just need to implement 'function getImage(key)' in
your parent activity.
smallnumbers, ported activity

This activity is based on gletters but uses images instead of text.

Recreated a new background image from http://openclipart.org