v2.2.1

Changes:

• Add get_goo_value
• Add cancel_sell_order
• Add cancel_buy_order
• Add sell_item
• Add buy_item
• Add SteamPrice
• Add Histogram
• Add PriceInfo
• convert_steam_price removed. Use SteamPrice magical methods ₊˚.⋆
• Order price will now return a SteamPrice object instead str
• get_item_histogram will now return a Histogram object instead a dict
• get_item_histogram will now raise MarketError when fail
• QA Fixes