Added sign editing functionality (If you want to update the price of an item, create a new sign and put the new price on the bottom line, leave the other 3 lines blank, and click with black dye. You can modify the description, the price, and the type of sign this way. Blank lines are ignored. You cannot, however, change a Device sign to a Buy sign, as the operations are incompatible with one another.)
Added two new permission nodes to go with Copy/Paste functionality (SignShop.CopyPaste [To edit your own signs] and SignShop.CopyPaste.Others [to edit other people's signs])
Fixed error with players being unable to sell to a shop if they had a low balance
