Cleaner SQL joins

This still isn't perfect.  It's a heck of a lot cleaner for simple cases though. 

Example test:

```Python
expr = join(tname[tname.id > 0], tcity, 'id')
result = compute(expr, ns)

assert normalize(str(result)) == normalize("""
SELECT name.id, name.name, place.city, place.country
FROM name JOIN place ON name.id = place.id
WHERE name.id > :id_1""")
```