Querying by part of message body

This feature allows user to query for messages or collections containing messages which in body of a message contained text passed by user.

To execute request in which user wants to find messages with "test failed" string XMPP client needs to add following element

<query xmlns="http://tigase.org/protocol/archive#query">
    <contains>test failed</contains>

as child element of @retrieve@ or @list@ element of request.