Minor tweaks after Ouyang's update

Added "Bounty" (how much a kill is worth) and "MAXHEALTH" to alien
class, to make up for the differences between alien and mothership.
Mothership health bar now works again.
Removed shipSelect function.
Renamed "Options" class to be "Powerups" to reflect their purpose
Deleted (and removed from the script list) unused files --
mothershipentity.js, collision.js, objects.js.
Minor css tweaks.
Added in more comments throughout the files.