API needs update

[damianac]

For example, buy_item route requires app_id parameter now and method for buying items is no longer functional until this gets added.

[Demitor]

The parameter app_id in buy_item is optional and will default to CS:GO (730) according to BitSkins API documentation https://bitskins.com/api#buy_item.

The library will send this parameter with every request and will default to CSGO (730) if you do not supply another app_id when initializing the API class.