Create Matrix Like screen using Ruby

the idea is that this screen is a vertical lines each line
moves vertically in random speed.

each line has number of entities, each entity has a random
character with random size.

each character is will be changed randomly,
so we created an entity class with this features
then created the vertical lines of them has the same number
of characters but with different speeds.

and let them roll..
after creating it i noticed that it changed to new character
by 60 change per second (the FPS) so i had to invent a small technique
to skip some second for the character changing.

i could have been changed the FPS to lower value but this will slow
down the lines movement, so to keep lines moving smoothly i kept FPS
high anad skipped random number of frames using a entity class member
updatelimit and lastupdate

Gist: https://gist.github.com/9584057