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)

Actually come to think of: all that's actually needed is access to onActivityResult(...) for Android. Would it be OK to add a addOnActivityResultListener(...listener...) to AndroidApplication.java? I'll remove the Java reflection from Android and move it into PurchaseSystem.java. When PurchaseSystem is created it will use Java reflection to set up the corresponding stores.

It would still be plug & play. Just add the jar files and it would all work out of the box.