Added ItemTooltipEvent

This event is fired at the end of ItemStack.getTooltip(EntityPlayer, boolean), which in turn is called from it's respective GUIContainer. It allows an itemstack's tooltip to be changed depending on the player, itemstack or whether the advanced information on item tooltips is being shown, toggled by F3+H.
Merge pull request #796 from HoBoS-TaCo/master

Added ItemTooltipEvent