EmptyCursor should never crash when asked for column index.

The smelly thing here is returning -1 from getColumnIndexOrThrow, but
it's necessary to make the EmptyCursor work with CursorAdapter, which
expects the BaseColumns._ID column to be present in non-null Cursors.
Merge pull request #6 from futuresimple/empty_cursor_columns_fix

EmptyCursor should never crash when asked for column index.