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)

I think that adding extensions into the core classes defeats the purpose of being an extension. The core classes should not have any dependency (actual or implied) on the extension - gdx-jnigen being an exception to that rule, of course.