memory, added rotation animation and dataset
memory, minor code cleanup (comments and indent)
memory, major refactoring

Now the memory engine no more analyse dataset content to find matching pair.

The caller of the memory now must provide a dataset that contains for each
level the list of images, texts and / or sounds.

Made all the math memory to be based on math table from 1 to 10.

Now the memory span the whole screen.

Improved scoring system, added an icon from openclipart to represent the
user.

Much simplified code for the memory core.
memory, implemented an AI for tux

The AI includes a random to make Tux fails randomly.
memory-wordnumber, ported from the Gtk+ version

The background imange comes from openclipart.org