Fixes heal flag bugs when player.maxHealth > 20

With the new Bukkit Changes the Player Health can be above 20, this fixes the magic number by using the now built in Bukkit functionality.

Examples of this bug were Health jumping to 1/20 hearts when having a heal-amount: -1
Merge pull request #270 from Silthus/patch-1

Fixes heal flag bugs when player.maxHealth > 20