Added option to undo strength potion changes

This should fix NerdNu/NerdBugs#140, this change removes the damage increase to Strength 1 and 2 potions. 

According to [this](http://www.minecraftwiki.net/index.php?title=Status_effect&oldid=509091) wiki page (1.5.2), and [this](http://www.minecraftwiki.net/index.php?title=Status_effect&oldid=512125) wiki page [1.6.2], strength 2 in 1.5 was simply 3 extra damage (or 6 extra damage, with strength 2), whereas strength 2 in 1.6 is a 130%/160% increase on base weapon damage. 

Bukkit does not have an attribute API finished as of right now. I did not see the purpose of including a way to change the attributes of the strength 2 potions using NMS and org.bukkit.craftbukkit, as those are more likely to change/break between versions. I'll look at reimplementing this when the Attribute API is released.

Thanks.