Add Unit class to PopGame
Enable units to build houses and farms
Add flag graphics to the house to indicate residents inside it
Units try to go to flat empty space to build houses

Also fixed isFlat() which has been returned wrong cell's value.
Merge two units into one if they get close enough to each other
Prevent stacking up frames when the script is paused

When we debug the game for a while, the game's frame time will build up.
We use constant simulation step to calculate game progression, so it
may have too many simulation steps in a large frame time.  We just limit
simulation step count in a frame under 10.
Correct farm area updates
Make mouse interaction much smoother

Adding mouseover event handler to all cells is somewhat inefficient.
Now we have single event handler for all cells.  It responds much better
at least in Firefox.
Delete Firefox only JavaScript syntax for compatibility to Chrome