Dynamic Difficulty and some fixes.

Dynamic Difficulty:
- Finishing a wave increases the difficulty level by 1. For every health
point lost in a wave, the difficulty goes down by 10%. (example:
Difficulty 10 (Wave 10), 2 health points are lost, next wave (Wave 11)
is at difficulty 8).
- The amount of ships each difficulty level are as follows: 1, 3, 3, 4,
4, 5, 5, 6, 6, ...
- The interval between ship spawns is 8secs at difficulty 1 and is
decreased by 40ms each level until it reaches 4secs.
- Ships and Row Boats start off with a maximum speed of 50 (50px per
second) which increases by 0.5 each difficulty level until it caps at
75.
- Infantry start off with a maximum speed of 10 which is increased by
0.1 each difficulty level until it caps at 20.

Fixed Power-ups not being removed properly in some cases.
The Freeze Time Power-up now pauses the Ship and Row Boat animations.