Added the ability to queue up some blocks and then calculate the experience in them!

Commands and permissions:

• /fxp add
  • Permission: furnacexp.fxp.add
  • Adds the block you are looking at to the block queue
• /fxp calculate or /fxp calc
  • Permission: furnacexp.fxp.calculate
  • Calculates the total experience stored in the blocks in the queue
• /fxp clear
  • Permission: furnacexp.fxp.clear
  • Clears your block queue
• /fxp clearall
  • Permission: furnacexp.fxp.clearadd
  • Clears all the blocks in the queue for every user.