Config to apply potions on spawn.

Closes #615.

The new APPLYPOTION configuration option allows warzone makers to
give potion effects ot players every time they spawn.

Use case: Resistance 5 for a few seconds on player spawn to prevent
them from being "spawn camped" - killed when they step out by other
players waiting nearby.

Format of APPLYPOTION: effect_type:duration:amplification

 effect_type:
  For a valid list of values that can be used for this type param,
  please see http://jd.bukkit.org/rb/apidocs/org/bukkit/potion/PotionEffectType.html

 duration:
  Time in ticks (1/20 sec) that the potion will apply for.

 amplification:
  Amplifier to the potion's power.

@taoneill @cmastudios documentate how to tune this option before
 1.9 release on the wiki. Also please test use of this option in
 game (pulling a @grinning here).