Check if lookup can operates on select

To get decrypted values we need to run a SQL like this one:

```SQL
SELECT pgp_pub_decrypt(fieldname, private_key) FROM table;
```

We might be able to achieve the same thing as the `annotate` with django lookup.

cc @maxpeterson 
Thanks, I've started from the example in the [documentation](https://docs.djangoproject.com/en/1.7/howto/custom-lookups/). I will update it soon.