Allow simultaneous use of multiple API credentials and modes #4

There is a problem when the application must simultaneously use the API with a variety of sellers (with different private_key, seller_id, username, password) and modes (sandbox/production).

I'd suggest not directly using the Api object and class methods and instead passing an instance of an Api object (with proper instance methods) to the Charge, Company, Contact, Coupon, etc. class methods.

I can work in this enhancement if anyone cares to merge it, as we definitely need multiple of our clients have their own 2Checkout API keys.