In the resulting SQL query, if there are an alias on the joined tables, these
aliases must be used in the ON clause (instead of the full table name).

For instance, the following ADQL query:

```sql
  SELECT *
  FROM tableA AS a NATURAL JOIN tableB AS b;
```

should be translated into the following SQL:

```sql
  SELECT *
  FROM tableA AS a
    INNER JOIN tableB AS b
      ON a.id = b.id
```

This commit complete the resolution of the Pull Request #16
(more details about the issue can be got in there)