====== chat ======
**C → S → C {type:'chat', from:..., to:..., format:..., msg:...}**

chat - text message. Client wanting to send a text message sends chat message to server, server forwards it to requested client.

arguments:
  * from - address message was sent from
  * to - address the message was sent to
  * format - message format:
      * "text/plain" - text message
      * "iscomposing" - person has started to type an answer
      * messages received from SIP clients may have other message formats too
  * msg - the message text

optional argument:
  * from_dn - display name of person sending the message

===== error =====
**S → C {type:'chat', from:..., error:..., msg:...}**

If an error occurs server can send chat message containing error description to client:
  * from - error origin address
  * error - error number:
      * 404: Not found - attempted to send chat message to an unknown recipient
      * other error numbers might also occur
  * msg - error description in english