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:

  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:

  [
    {{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}
Merge pull request #19 from sebastian/bugfix/type-spec-simple-query

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