Added trigger_bossclass

A new brush entity which turns the zombie who enters the volume into
their chosen preferred boss:

Keyvalues:
enabled - Enable the brush entity.
silent - If yes then then the server does not announce that a player has
become the new boss.
instantchange - If yes then anyone who gets swapped will instantly be
changed right on the spot.

Inputs:
enable(void) : Enable the entity.
disable(void) : Disable the entity.
seton(integer)  : Set Enabled keyvalue.
setsilent(integer) : Set Silent keyvalue.
setinstantchange(integer) : Set Change Instantly keyvalue.

Outputs:
OnBossTouched  : Set when a boss starts touching the brush. Is not fired
when a zombie becomes a boss in this volume.
Fixed angles-snap to (0,0,0) on forced classchange

Edited trigger_zombieclass so that it saves the original view angle of
the player