Adds support for affiliate and campaign tokens (under iOS 8)

iOS 8 added the ability to use affiliate codes. This pull request adds two properties to DAAppsViewController. `affiliateToken` and `campaignToken` setting a value for these will (under iOS 8) set `SKStoreProductParameterAffiliateToken` and `SKStoreProductParameterCampaignToken` on every `SKStoreProductViewController` shown by DAAppsViewController.

The pull request maintains support for iOS 7 in two ways:

1. the properties are only included if you target the iOS 8 sdk.
2. it checks for the existence of the iOS 8 only `SKStoreProductParameter` constants before referencing them. 

Let me know if you have any questions or suggestions for improvements to this pull request! :)