Added support for projections in GAE

Devs can now specify a list of properties that should be returned when doing a query, e.g.

Collection<String> projections = Arrays.asList(
                    DQuestionDao.COLUMN_NAME_ORDERING,
                    DQuestionDao.COLUMN_NAME_QUESTION,
                    DQuestionDao.COLUMN_NAME_THUMBNAILIMAGEURL);

this.getDao().queryPage(pageSize, DQuestionDao.COLUMN_NAME_ORDERING, true, null, false, projections, cursorKey);

Only implemented for GAE (not SQL or Android).
Note! In GAE the properties must be added to the index for projections to work.