Spring Break Commit

Breaking my refactor rules and adding things!
- New audio class that loads a low-passed version of each audio source
and uses it to make audio sound like it is being heard from a distance
- New sounds!  Starting to replace old sounds with cool new ones.
- Assault rifle sounds
- Wall-impact sounds for bullets
- Zombie death sounds
- Only the player shows floating damage values.  Other entities get
cool:
- Meat chunks!  Damaged hostiles spew chunks of flesh or something.
- Implemented new tiles: grass and wood floor!
- Currently just placed randomly in level (better levels coming soon)
- Fixed solid checking, so we can have various floor tiles
- Sparks fly when bullets hit walls
- Game can be paused and unpaused by pressing escape
- Fixed script loading since entity.js refactor.  The game no longer
randomly fails to load!
- Fixed Nyan Gun from really old versions.  Now available from Player
menu.
- The button that gives the player a RandomGun now pushes into the
inventory rather than overwriting slots.  If the inventory is full, this
function won't do anything.
- Todo: Refactor, level gen