Fixed remaining bugs of the iOS purchase manager and added some logging.

This PR fixes the remaining bugs. The iOS purchase manager has been thoroughly tested now and did work for me everytime.

To test the iOS manager just:
1. create a new app in iTunes Connect using your wild-card (*) bundle identifier and add the bundle id from the test app as a suffix.
2. Create a new in-app-purchase using a distinctive product identifier. Use something distinctive you won't use again (you won't be able to use that id again even if you delete the in-app-purchase!!!).
3. Update the gdx-pay sample and change the product offer id to the product identifier you used in step 2.
4. (optional) Create a test user for your country. Use a distinctive (non-existing) email address (you won't be able to use that email address again with Apple products even after deleting the test user!!!).
5. Log out from your apple account on your iDevice.
6. Launch the gdx-pay ios test app. When asked, login with the credentials you created in step 4.