FELIX-4075 Fix Configuration ZIP writing

- Consolidate ConfigurationWriter classes into a new
  helpers package (out of the AbstractWebConsolePlugin)
- Create a dead-simple SimpleJson helper class to
  generate the configuration ZIP index.json file
- Create an index.json file providing the timestamp
  and the files provided by the plugins
- Use "<pre>" formatting for plain text and JSON
  output in the HTML (Browser) context

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1486594 13f79535-47bb-0310-9956-ffa450edef68
Update changelog

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1486599 13f79535-47bb-0310-9956-ffa450edef68