Changed Damageable.java to a reflect the 1.6 health changes in a more compatible way.

Affected by the previous way are dozens of plugins that use CraftBukkit methods.
While those aren't using supported API, there's no reason to make it impossible for them to update without switching to Maven, overturning their entire build process (or converting some obscure plugin to another build system, just to make a plugin for a game work)