Implement S-codes (and allow to search by them)

```
Implement S-codes.

See issue 70. S-codes are a better alternative (than A-codes) for searching. 
Each S-code will describe a set of conditions. Users should be able to join and 
mix these conditions in a query.
```

Original issue reported on code.google.com by `rygielski` on 28 Jun 2013 at 7:46
