boshclient.py example doesn't always authenticate successfully

The boshclient.py example is buggy because when examining the response from the XMPP server, it only looks at the first child of `<success>` for the `<bind>` element.  However, `<bind>` might not be the first child; for example, in the XML returned from Prosody 0.8, it's the second child.

This commit fixes the code to look at all the children of `<success>` for a `<bind>` element.