Added feature to load partial trees, as used by Sencha

  but not stored in the DB yet

  Beautified code, removing duplicated code

  Fixed issue that error messages weren't passed back to the server correctly

  Fixed "GanttEditor: Error saving data on the server side: No authentication found ('')" bug.
  Apparently Chrome sometimes sends out PUT XHTTP calls without cookies.
  So ]po[ then correctly identifies a non-authenticated session.
  As a workaround, I have added auth_token authentication in the SenchaPreference store,
  so that the calls are authenticated, even if the cookies should disappear for some
  reason.

  - Added CostCenter field
  - Added CostCenter model and store

  - Improved error message
  - Added remaining columns to GanttTreeStore (material)

  suitable to be parsed by Sencha ExtJS