Postgresql: .pgpass support

[hackenspp]

Connectorx does not support passwords stored in the .pgpass (or pgpass.conf in windows). It's useful to avoid to specify it directly in the code or some sort of config file.

[aimtsou]

This can be achieved, I need to think how it can be given from the user that we use postgres in the conn variable. After that we need some OS dependent code to see if pgpass exist and then to see if we read it how it gets passed. I believe it is the rewrite_conn method but I will need to dig deeper.

[surister]

To some level we need to introduce some mechanism to do X depending on the database and the ability to identify the database (by uri.scheme I suppose)

This would also enable features like this to be easily implemented for other databases

[surister]

@wangxiaoying Can you asign this to @aimtsou