Adding populators from SkylandsPlus+ (GPL) through maven.
Support SkylandsPlus populators, when installed

Detects the SkylandsPlus plugin when available and uses its populators. Note that
the bukkit and craftbukkit versions should match between plugins.

Also refactored the methods for getting available plugins to instance methods.
Updated README
Merge pull request #1 from sbliven/master

Add SkylandsPlus support