Create Multiple style of levels

[FranciscoKnebel]

There are already color definitions for backgrounds, but sets of levels should be themed, related to #9.
Different amounts of monsters, with different speeds, and with independent movement between groups could also be implemented.

[FranciscoKnebel]

BuildLevel module 4fd0451
Random weighted choice f713344
The idea is to define multiple themes, where each theme contains multiple levels, with type of enemy it can spawn, with weighted random choice (stage defined probability to spawn a certain type of enemy)