Add unimplemented guns

[cxong]

From the code, there are guns that are not fully implemented:

• Heatseeker
• PulseRifle

Heatseeker fires a slow moving medium range red projectile, and has very poor DPS. However, its namesake suggests that it seeks targets, which it actually doesn't.

PulseRifle is an extremely fast firing and long range repeating weapon, like the machine gun but better all round. It may be unbalanced and the muzzle location is broken, but could be an interesting gun by a boss.