minor tweaks to better support using CursorAdapter

- allow buildSpanForObject to be overridden, so that Cursor can be
  translated in to an object
- make TokenImageSpan public, required for return type of ^
- check to see if tokenSpan.getToken is in objects, not selectedObject,
  in case they're not the same. shouldn't affect anything when they are
Merge pull request #33 from ross/cursor_support

minor tweaks to better support using CursorAdapter