Global cache

EDIT: no more singleton class!

Two main changes:

- add a Cache class which is a singleton. PLEASE review this. I know that singleton classes are generally bad style, so I want to make sure that changes here won't screw things up. The issue with how I was doing it previously was that it would re-create the cache each time you come back to the activity.
- remove unused activity in manifest. I'm hopeful this will solve our logging issues, since I made the corresponding change in the Facebook dev android panel