Fixed bug with prop nailing.

Nails do not appear when you nail a prop. The prop freezes in place but
does not gain additional health, you can not heal the prop, and humans can
destroy the prop.

The bug is caused by: 3e40769454c65a425f6847d40e63100901a765a1

Specifically it is caused by, entities/entities/prop_nail/init.lua#54, as 'self.HealthOveride' is nil.
This is caused by entities/weapons/weapon_zs_hammer/init.lua#203:
'nail:Spawn()', being after the call to 'AttachTo'. The nail's
initialize function is not called until it is spawned which means
'self.HealthOveride' does not yet exist.

I am not sure if spawning the nail before attaching it will cause problems but it seems to work fine when I tested it.