Typespecs added

See #9.

Maybe also add exported typespecs for messages, sended as reply from `apgsql` and `ipgsql`? Like
```erlang
-export_type([squery_msg/0]).

-type squery_msg() :: {pgsql:connection(), reference(), {columns, [#column{}]}
                      | {pgsql:connection(), reference(), {error, #error{}}}...
```
Which are not used in interface functions, but may be used to simplify typespecing user's code. Thoughts?