Extension: libGDX Pay API.

Given there has been no progress, I have converted my old draft for the libGDX payment API to a libGDX extension. The extension isn't linked into the libGDX build system so far. Also, this should probably be a branch? In any case, this is a draft for the pay API extension, so I am looking for feedback. 

FOR USERS:
To see how it works for a user of the payment API, have a look at "PurchaseManager.java" (Javadoc part explaining it all how to create a new payment service and make purchases).

FOR CONTRIBUTORS/LIBGDX DEVELOPERS:
libGDX contributors (e.g. contributing the IAP for Google Play, iTunes, Amazon etc.) will need to implement the PurchaseManager.java and fill in the 2 functions in there. Have a look at my "PurchaseManagerGoogleAndroid.java" on what would be needed. It's still empty.


Anyhow, the purchase API is pretty bare-bone. I think in the long-run we should add an Inventory.java on top of the libGDX Pay API which keeps track of purchases. I am flexible what naming concerns. It's possible that I got some things wrong. I believe there might be some people with a lot more experience so I'd be glad to address any concerns you guys have. Please note that I tried to simplify several things from my previous pull request (from last year). :-D