Add copy() method to Wave interface.

When the WaveManager is queried for the next wave to be spawned via
the next() method, it now returns a copy of the wave instead of the
wave itself. This is because waves (boss waves in particular) have
state, and this state is transferred to the next occurrence of the
given wave. This caused recurrent boss waves to inconsistently share
state, which resulted in the BossAbilityThread bailing early after
the first occurrence of the boss wave it belonged to. By copying
the initial wave state in the WaveManager, the issue is fixed.

Fixes: http://dev.bukkit.org/bukkit-plugins/mobarena/tickets/1220/