And() & Or() override inner Cond elements relationship

``` go
cond1 := db.Cond{"col1": "val1", "col2": "val2"}
cond2 := db.Cond{"col3": "val3"}

q.Where(db.Or(cond1, cond2))
```

expected:
`WHERE (col1 = val1 AND col2 = val2) OR (col3 = val3)`

actual 
`WHERE ((col1 = val1 AND col2 = val2 OR col3 = val3))`

The same happens with And
