Added Firing alien stub

This is the stub for adding firing aliens.  It is also a sort-of test
for branching!
Added shot logic to be able to damage ship and for the player to die.
Added Firing logic.  Firing is now successful.
Added Ship Death effect, reset starting level to zero.  READY to
merge!!!
Finalising some changes. READY to merge!
Merge branch 'Firing-aliens'