Add page type constants to SDK

[fairlane]

The SDK is missing all the default page-type constants and these could be added to the SDK

const PAGE_TYPE_FRONT_PAGE = 'front';
const PAGE_TYPE_CART = 'cart';
const PAGE_TYPE_PRODUCT = 'product';
const PAGE_TYPE_CATEGORY = 'category';
const PAGE_TYPE_SEARCH = 'search';
const PAGE_TYPE_NOTFOUND = 'notfound';
const PAGE_TYPE_ORDER = 'order';

[mridang]

This isn't really needed as they would be hard to reference from Magento's layout XML anyway.