Mostly kinda sorta fixing IMMUTANT-197

Not all XA drivers support connection creation via URL, postgres in
particular (its non-xa driver does). I considered parsing the URL
myself and extracting the host, port, db, user, and password from the
postgres URL, but because the URL supports so many other options that
wouldn't be used, I felt it might lead to confusion from unmet
expectations.

Pretty much all the other Immutant-supported adapters do support
connection URL's, though most Oracle drivers appear not to include the
user/password.