Stacking made possible again

Stacking is done with sprites:
>> after selecting a unit, pressing the space bar when the mouse is over
another unit will add that unit to the selected unit's stack
>> right clicking on a unit which is stacked will cycle through the
units in the stack
>> shift right clicking on a unit which is stacked will remove units
from the stack
Merge pull request #17 from harr6176/master

Stacking made possible again