Adjust return type spec of simple_query/{1,2,3}

A simple query that performs multiple actions
result in a list of results. The current spec
definition does not reflect this.

An very artificial example would be:

```erlang
Sql = [
  "CREATE TABLE table1 (n integer);"
  "CREATE TABLE table2 (n integer);"
  "DROP TABLE table1;"
  "DROP TABLE table2;"
],
pgsql_connection:simple_query(Sql, C)
```

Which returns:

```erlang
[
  {{drop,table},[]},
  {{drop,table},[]},
  {{create,table},[]},
  {{create,table},[]}
]
```

This does not correspond to the return type of result_tuple(),
but rather [result_tuple()], like is used in batch_query/{3,4,5}