Add support for SASL failure 'text' elements

- Also move toString() into TopLevelStreamElement.
- Fix SASLFailure toXML xmlnsAttribute(NAMESPACE)
- Improve SASLFailure parsing
- And introduce XmlUnitUtils
Add support for SASL X-OAUTH2
Support for XEP-0141: Data Forms Layout.

Data Forms layouts are a part of Data Forms and implemented as
extensions, added to a Dataform. Fixes issue SMACK-612.