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)

another idea:

* we add an option to **AndroidApplicationConfiguration** such as "enableIAP". If false that code segment using Java reflection would never be executed.
* also, I would need to add the code to AndroidFragmentApplication (AndroidApplicationBase wouldn't work because it's an interface).

In any case, in both cases no funny business would take place if IAP is disabled via the configuration file.