Allow catching mobs for free with a permission

The major feature of this pull request is to be able to catch mobs without a fee if you have a certain permission node (eggcatcher.free). The server owner of the server I play wanted to be able to have catching mobs be free in his town worlds, so by setting this permission as a world-specific permission on the town worlds, this is now possible.

I also did some other minor changes. You can choose to leave these out if you don't want them. (I assume GitHub lets you do so, anyway, pardon me if it doesn't -- I've never received a pull request before):
* I couldn't build with Maven when the "maven-shade-plugin" had a version specified, so I removed the version specification. Someone else had also had this problem, I found the solution described here: https://github.com/Parquet/parquet-mr/issues/125#issuecomment-22554047
* Target 1.6.4 (which is what our server runs)
* Switch from item ID-based ItemStack constructor usage to Material-based ItemStack constructor, as the item ID ones are deprecated.
* Replace usage of wrapper class Boolean with primitive boolean. Java might be smart enough on compilation to do this for you, but in case not, using the primitive is slightly more efficient, as (un)boxing is avoided.
* During the maven build, I shade org.mcstats to me.shansen.EggCatcher.mcstats. In the case where another plugin is also using org.mcstats.Metrics (and also didn't rename the package), this will prevent a package conflict.