Restore buyed items from Appstore and Androidx86 support

The restoreTransactions method retrieve the items from the appstore when you send the parameter "true" or keep the original unusual functionality (Based in the @tsangwailam fix).

InAppPurchase.getInstance().restoreTransactions(true);

And the @jhansen fix:
BillingActivity uses its own iapHelper to better handle closing of the activity.
Added event to indicate when async init() has completed.
Added Android-x86 support.