Add support to hide ads using an in-app purchase

[breber]

This is mostly implemented in the inapp_billing branch.

A couple things that need to be done:

• Figure out where the best place to put a button that allows them to initiate the in app billing
  • Right now, the About button on the main menu will do it
• Add a callback method
  • RestoreTransactions might need something
  • RequestHideAds
• Hide ads in all Activities that currently display ads if InAppBilling.isRegistered == true
• Figure out how much we want to charge to hide ads

[breber]

http://developer.android.com/guide/google/play/billing/billing_testing.html