Some bug fixes + Health orb system

-Sentry moving keys changed to A and D (E is in conflict with the
taunting in Gauntlet missions)
-The absorption shield strength of the player wasn't resetting correctly
after the start of a mission - fixed
-Healing orbs - have a chance to appear after killing an enemy - moving
your plane over them heals your for 5hp; there can only be 3 health orbs
at once on the screen