Misc zPermissions fixes and changes

First off, I had to bump the Bukkit version in the POM to 1.5.1-R0.3-SNAPSHOT to get things to build with Maven. This change isn't part of my pull request, since I wasn't sure if it was just me or what. But for whatever reason, version 1.5.1-R0.2-SNAPSHOT from the Bukkit repo was always ending up corrupted.

Anyway, this PR does a number of things:

*   Fixes setting world-specific permissions in zPerms.
*   Adds an alternate way for admins to determine the primary group when using zPerms.
*   Lowers the service priority of the zPerms Vault plugin from Highest to High to make it easier to override things from my side, for testing and what not.

Thanks!