0.6.7 Spawners

# Spawners
- EnemySpawner.cs: spawns enemies in a queue (List<Enemy) while there
are enemies to spawn at a given interval and while the spawned flag is
checked.
- EnemyPortal.cs: inherits from EnemySpawner, animates it’s sprite
while spawning, dying, and idling while spawning enemies. Removes
itself from play when it runs out of spawns.
- Galaxy.cs now tracks spawners in addition to enemies and doesn’t end
play until both enemies and spawners are out of the game.

# Bug Fixes
- Fixed bug in Station where the player could fire a targeted weapon
without a target and an error would be thrown.
- Fixed bug in Station where the station tried to look at removed
Enemies.
- The Galaxy now properly starts itself, and inPlay is no longer a
static field.