adding some networking helper classes
add docu to NetworkUtils + fixes
add ModifiedInfo to Thing
fixing some constness
add netio::Response
tweaking and expanding network_utils

especially ContentType
cleaning up and streamlining netio::Network
adjusting netio::Error to recent changes

todo: need to change some more stuff here
add netio::Message::clear()

todo: probably don't need this one any more. merge its features into
netio::Error
adjusting Manager to recent overall design changes
add ManagerTests

and configure option for oqtocat-test-user password
fix NetworkTests
don't add probably configured user password
removed netio::Message

going to merge it into netio::Error
merged netio::Message features into netio::Error

plus further functionality
adjusting other stuff to recent Message-Error merge
fix global includes

also added general JSON parsing function

todo: always use this function for JSON parsin
replaced manual instanciation of QJson::Parser

with recently added global helper function oqtocat::parseJson
adding Manager::getThing test; discovering bugs and fixing them

remember: QNetworkReply::readAll() can only be called once; a second
          call will return an empty string.

also: make sure to overload QDebug operator<<(QDebug dbg, const TypeT& t)
      for all custom classes, structs, types and enums.