enemy presence reverts land

redux of https://github.com/MassiveCraft/Factions/pull/191

If a player moves onto enemy faction land and the enemy faction does not have enough power to keep it, the land is automatically reverted to wilderness. The player is awarded money from the enemy faction for the reversion.

As with https://github.com/MassiveCraft/Factions/pull/265 you'll have to ignore the travis build error below until gradle and travis files are added.

This is now running on `llamacraft.com`. Feel free to go there and test it out.