Allow user to take control of how to connect to the database provider.

This specific change was done to allow for socket based connection to postgres provider. The current version of the code assumes that "url" is required to make connection to the database.

Everything works as expected unless the host is actually a proxy and furthermore its a Unix Socket. This allowed me to run the plugin within google app engine while making connection to postgres via cloudsql proxy. When running proxy the socket address that gets created looks like:

/cloudsql/xxxx:xxxxx:xxxxx

This change also allows me to specify pooling setting, that I can use for the purpose of my plugin code.