Clarify usage, use real namespace for examples and add missing section:
* Clarify usage of trust messages by protocols such as Automatic Trust Management (ATM)
* Use namespace 'urn:xmpp:atm:0' of Automatic Trust Management (ATM) as example for 'usage' attribute.
* Add section 'Security Considerations'
Improve explanations, descriptions and examples, introduce new attribute and complete all sections:
* Remove link to encryption protocol namespaces
* Add short name
* Shorten and improve introduction
* Use emphasizing text formatting instead of quotation marks
* Add new section for explaining the core properties of trust messages
* Add examples comparing trust messages to public key certificates
* Improve description of trust message structure
* Introduce 'usage' attribute for 'trust-message' element
* Focus on XEP-0420 and adjust examples accordingly
* Complete sections 'IANA Considerations', 'XMPP Registrar Considerations' and 'XML Schema'
