Add converting framework as well as the converter for Backpack

Adds command /pvconvert <all | plugin name>
Adds permission playervaults.convert
Adds Converter interface for future converters
Adds BackpackConverter - A converter for Backpack
(http://dev.bukkit.org/bukkit-plugins/backpack/)
Make @evilmidget38 happy
Merge pull request #39 from turt2live/BackpackConverter

Add converting framework as well as the converter for Backpack