Bugged enemy test

First attempt at adding enemies. Many things wrong.

Foremost is could not find how to process an array of enemy objects in
javascript. I do not yet know how to cast an array elem back to its true
object type, therefore JS complains that the function for a type object
does not exist, though it exists for type enemy.

Second, enemies seem to gain distance inverse of what is expected.
Walking towards the enemy makes it appear to shrink, and vice versa.

Aside from the above headache, the enemy class is fleshed out, enemies
can have position, and are rendered.

Todo: Fix bugs, add collision/dmg/scoring, improve a bit.
Then: Draw weapon pickups and objective pickup
Fin: Add sound effects and publish ver1