Add functionality to return IAB feature support

[Vassili Philippov]

Some appstores support subscriptions in IAB and some do not.

Some appstores support non-consumable goods in IAB and some do not.

Different appstores support different intervals for subscriptions.

Since OpenIAB lib wraps and abstracts IAB logic we need to add functions that will describe appstore capabilities. Like isSubscriptionSupported(), etc

Best regards,

Vassili