Add listKeys method to query object. Discussed in #62.

I didn't want to refactor the AbstractQuery class and children to pass
the key type so this ugly method is required...

@SuppressWarnings("unchecked")
public <K> List<K> readKeys(Cursor cursor, boolean close) {
	return (List<K>) dao.readKeys(cursor, close);
}

To use for deletion it looks like this:
dao.deleteByKeyInTx(dao.queryBuilder().<Long>listKeys());

I grant greenrobot ownership of the code changes contained in this
commit.