Fixed 2 bugs:

• Enemy generation wasn't choosing types correctly--it was far more likely that the default alien would appear than it should have been.
• If the heat alarm was sounding at the end of the level, it kept sounding in the next level.

Known bugs:

• Number pad keys will fire multiple shots in quick succession (or maybe this is an issue with my hardware)
• Prime bomb rays will eliminate all primes, including those they haven't reached yet
• Level indicators won't display correctly after level 5
• Game will stop after level 9