- Add a `Stanza` class which can be used by Strophe because it has a
`tree()` function. This is what gets returned by the `stx` tagged
template.
- Throw an error when no valid namespace is on the stanza.
Strophe.Builder used to automatically add the `jabber:client` namespace,
but that doesn't happen with `toStanza`, so we need to fail if it's not
specified by the user.
- Use the Strophe XML Parser
This opens the door to NodeJS support
