Authentication lookup helper

Usage

get_clowderauth(key, user, pass, url, authfile = "~/.pecan.clowder.xml")

key, user, pass: passed unchanged from download.Geostreams call, possibly null
url: matched against <hostname> in authfile, ignored if authfile contains no hostname.
authfile: path to a PEcAn-formatted XML settings file; must contain a <clowder> key