Removed necessity to subclass SugarApp

Like described in #44 it is sometimes not feasible to subclass SugarApp. In that cases it would be better to have some kind of a static initialiser do get things running.

In the implemented variant it is just needed to initialize Sugar by calling `SugarApp.init(Context)` when the app starts up. To close the database we also need to call `SugarApp.terminate()` to indicate that the database should be closed.