Define Cart Methods

[stresslimit]

Return values for getCart and emptyCart are simple enough, but for addToCart, removeFromCart, updateQuantity, etc, what should we return? Do cart update methods just return success/error, or the whole cart object again? I'm tempted to return the whole cart again so that we don't have to make another trip to the server to update cart. Options:

• if we return success/err, we have to then go back to the server again to get the updated cart
• if we return the new cart object, we save this extra request. If we don't want to change Phoenix, at least the library can do the 2 calls for us and make it simpler to implement