Add property similar to "description" to the DB wrapper

[Cito]

In #39 we added a method fieldinfo() to the query in the classic module, which takes an optional field name or number. If nothing is specified, the field info for all fields of the query is returned.

This method is a bit low-level though, the provided information is pretty internal and difficult to interpret. On the DB wrapper class, we may want to add another higher-level method or property similar to the description attribute in the DB-API 2 module. This could be a little bit more informative and PostgreSQL-specific than the DB-API 2 description, but the idea would be the same. We could even reuse much of the code of the description property in the DB-API 2 since it is also based on a lower-level method similar to fieldinfo().