Added convenience fields to Postgres foreign key query results

This extracts the referencing column, referenced table and referenced column from the constraint definition string returned by Postgres and puts them in their own fields making it more consistent with results returned by other dialects when using the query returned by `getForeignKeysQuery()` particularly SQLite's `PRAGMA foreign_key_list(tableName)`.

So instead of returning:
``` javascript
[ { constraint_name: 'my_fk',
    condef: 'FOREIGN KEY (user) REFERENCES users(id)',]
```
It now returns:
``` javascript
[ { constraint_name: 'my_fk',
    condef: 'FOREIGN KEY (user) REFERENCES users(id)',
    id: 'my_fk',
    table: 'users',
    from: 'user',
    to: 'id' } ]
```
You should probably call the function :)