IAP: gdx-pay extension for InApp payments 

I managed to get a first draft for the gdx-pay extension (cross-platform InApp payments for libGDX) ready for review. This is less of a pull request but more of a request for feedback. Please comment - Thanks :-D

What's currently implemented (extension projects):
* *gdx-pay*: the cross-platform code (**complete**)
* *gdx-pay-android-openiab*: gdx-pay implementation for Android supporting Google Play, Amazon, Samsung and a few other stores. I used the open source OpenIAB library. (**complete**)
* *gdx-pay-iosrobovm-apple*: stub for the iOS store (**needs implementation**)
* *gdx-pay-desktop-apple*: stub for the Apple Mac store (**needs implementation**)
* *gdx-pay-gwt-googlewallet*: stub for the GWT backend/GoogleWallet (**needs implementation**)

This test demonstrates how the payment system is used in your platform-independent code:
* [PayTest.java](https://github.com/libgdx/libgdx/blob/b95748490445ce4c76b50146949d76ea84ceb2a0/tests/gdx-tests/src/com/badlogic/gdx/tests/PayTest.java)

To integrate into Android, you will need to add corresponding hooks to your activity (look for the 3x "// InApp:"):
* [Integration Android](https://github.com/libgdx/libgdx/blob/b95748490445ce4c76b50146949d76ea84ceb2a0/tests/gdx-tests-android/src/com/badlogic/gdx/tests/android/GdxTestActivity.java)

Screenshot running PayTest.java on an Android device:
![2014-09-14 libgdx - gdx-pay extension](https://cloud.githubusercontent.com/assets/2363626/4263061/a43b3e26-3bc7-11e4-8d5b-e6b5e9fb858c.png)

LifecycleListener doesn't forward onActivityResult(...) data. Besides LifecycleListener one would have to hook into the AndroidApplication to also get access to onActivityResult(...)...

> ... *adding* extensions into the core classes defeats the purpose of being an *extension*.

It depends how you define "adding". The gdx-pay classes are not added to the core. Also it depends on how you define an "extension". Shouldn't an extension be something that can be added easily, e.g. without any code changes? Just plug & play? 

Other suggestions?