Update messages_pl.properties

Update Polish translation.
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Date: 16.04.2013

Changelog:
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* Fixed:
antiBuildBreak (13)
antiBuildCraft (14)
antiBuildDrop (15)
days (69)
giveSpawn (134)
heal (146)
hours (164)
minutes (247)
noChapterMeta (281)
noMetaFirework (292)
noMetaPerm (293)
orderBalances (319)
survival (414)
playerUnbanned (521)
noPotionEffectPerm (524)

* Added:
uptime (469)
balanceOther (526)
pWeatherCurrent (527)
pWeatherNormal (528)
pWeatherOthersPermission (529)
pWeatherPlayers (530)
pWeatherReset (531)
pWeatherSet (532)
WeatherInvalidAlias (533)
cannotStackMob (534)