Updated Item sell Visual prices 1

[FEATURE] Now the items added in a selling inventory  will display information about the price a store will pay for them.
[FEATURE] Clicking with left mouse on a selling block with an item in hand will display the price for that entire stack.
[BUG] Selling an object to a store that has not enougth money to buy it will result in the item disappearing.

TODO - Interaction with ShiftClick, right click and dragging with the new Sell inventory.

-- Code related changes
[CODE] Rewritten static method Shop.sellToStore
[DOCUMENTATION] Added documentation for static methods in Shop class.
[CLASS] Added SellInventory and SellInventoryListener. The first class is a wrapper for the inventory, useful for displaying custom lores on items while maintaining a pure version of that items. Implementing the Inventory interface was impossibile, because it uses Craft-classes that are not good for plugin development because are of craftbukkit. The listener purpose is for Inventory signals that can interfere with the SellInventory funcionalities.