fixed key command parser in Parser.hs

add FallingGun object and all helper functions (smart constructor,
object function, renderObject case, recognizer)

modify gun object behavior to deal with collision with another gun
(set acceleration to 0)

modify hits in Main.hs and colliding in Object.hs to detect gun
collisions and only generate hit event for one gun in a collision

add hasGun flag to OOSAlien in Object.hs and modify alien behavior
to generate a falling gun when appropriate and modify renderObject
for aliens

modify game arrow return type to include current number of guns so
that successive rounds of play remember current number of guns

modify noAliensLeft in Main.hs to also check for falling guns (keeps
new round from starting before a gun has finished falling)