New update notifer to comply with BukkitDev rules

A new update notifier, based on Gravitylow's reference implementation, has replaced my homebrewed one. The new one uses Curse's API to check for new files. It's much better!

I have also begun to use the Apache Commons libraries available to plugins, lang and IO. These have replaced a few helper classes that I had previously written myself.

Version's .newerThan method has been replaced with a standard .compareTo method, as part of my upgrades to the update notifier.