Difficulty setting

[ForNeVeR]

There should be an ability to select / set the game difficulty.

Currently I think it should affect the game speed.

Also this task involves score table changes, because (as I think) we should have separate tables or sections for different game difficulties. Also affects #108 but I hope noone has touched that task yet :)

[rexim]

We could also adjust the game difficulty with the different AIs, probably? >.> #72