Plans in Site Creation: Create functionality to add domain with a plan to a cart

[staskus]

Update WordPressKit-iOS to support adding selected plans together with the domain.

To add a plan to a cart via me/shopping-cart/siteID we need to provide an object containing product_id of a plan. Example:

var planDictionary: [String: AnyObject] = [
    "product_id": 1003 as AnyObject
]

let parameters: [String: AnyObject] = [
    "temporary": (temporary ? "true" : "false") as AnyObject,
    "products": [productDictionary, planDictionary] as AnyObject
]