Misc. changes and fixes, see full description

Made store look better. Text looks better against lighter background.
Text turns red when hovered over if buyable.

Standardized code for store buttons, added to it.

Got rid of some variables and methods that were no longer being used.

Made HUD powerups line up correctly.

Made rocket fire more slowly.

Store is now a pickup and will kill other objects if it spawns in them.

Gave Megan credit for coin sound.