Draft for libGDX Pay API.
Purchase object improved to accept both an integer or String in the constructor for the item identifier.
Merge branch 'master' of https://github.com/noblemaster/libgdx into iap
added gdx-pay to build
Merge branch 'master' of https://github.com/libgdx/libgdx
Merge branch 'iap' of https://github.com/libgdx/libgdx
Pay API:
 - removed PurchaseObserver - there is now only 1 single listener: PurchaseListener
 - itentifier changed to String (from int).
Merge branch 'iap' of https://github.com/libgdx/libgdx
Added payment processor stubs for Google Wallet/GWT and Stripe/Desktop.
Merge branch 'iap' of https://github.com/libgdx/libgdx