v0.2.4 Update

Update includes:

1 new unit: Shark, an expensive magic resistance tank unit.

1 new game mode: Survival. A first pass attempt at adding randomly generated challenges to the game. In survival mode, a random set of units is generated, and the computer randomly generates an opposing army using that set. You get less money than the opponent, but can strategically counter their army. The next round, the opponent builds a completely new army from a new set of units while your units from previous rounds carry over and will aid you in the next battle. As the rounds go on, your army size will grow larger and larger -- see how far you can make it with good strategy and a little luck!

SOUND EFFECTS! There are now sound effects for units in combat. Next up will be music and sounds for the UI.