Set database on socket string connection

Allows to connect to a specific database trough this ways:
```
pg.connect('/some/path database', callback);
pg.connect('socket:/some/path?db=database', callback)
pg.connect('socket:/some/path?db=database&encoding=utf8', callback)
```

Closes [issue 484](https://github.com/brianc/node-postgres/issues/484)