Add random acceleration to loot drops from enemies

[cdsupina]

Currently, loot drops from enemies spawn at the location of the destroyed enemy and travel straight down. Let's give the loot a random 2D velocity vector in the positive y-direction. This will make the loot shoot upwards out of the destroyed enemy and then adjust its velocity to travel downward. This will have the benefit of:

• Giving the player more time to react and position to collect loot
• Making the game more visually appealing and interesting