Untested chain lightning ability added.
Tell the player if they are editing an existing class without specifying
the override flag.
Fix NPE resulting when wolf owner is not null during cleanup.
Fix bugs:

- NPE on using ChainAbility due to uninitialized HashMap.
- Snares not removing properly from arena.


Externalized cooldown for Chainability, and reduced cooldown time from
30 to 20 seconds.
Fix snare and chain abilities.
Update plugin.yml and pom file towards next pre-release.