Added health bars and non-square sprites

The health bars are rendered over everything else. Currently they are
shown for six seconds after its mob has been damaged. The colors of the
health bars are calculated by comparing the max health of a mob with its
current health.

The yellow bar is a feature which may later be used so that the mobs can
regain some of their lost health.

Non-square sprites have also been added.

I accidentally rolled back this commit so I rewrote the code. Hopefully
there are no bugs in it, but something may have been missed.
Added random levels

The idea behind the random level generation is to create rooms connected
with paths. This is accomplished by randomly placing rectangles on a set
range. By these rectangles some are randomly removed. The others get
connected with paths. Based on the rooms/rectangles, an array of tiles
is generated. Where a room or path was, a grass or flower tile is placed
(grass: 8/9, flower: 1/9).

Mob spawning has yet to be implemented on the random levels.

The mobs are now randomly spawned on non-solid tiles. Also, the Level
class now clears its list of mobs by itself. This prevents concurrent
modification exceptions to occur and removes the bug where some mobs did
not tick due to lists being changed.