Allow for common and host-specific config files.

Several tasks:

• Move configuration into a separate subdirectory, e.g. conf/ (same concept as for the log and status files, see #5 (closed)).
• Provide a layered configuration file schema:
  • An OPTIONAL site-generic configuration file config.common.xml to provide settings that are not host specific (email config, notification settings, ...).
  • Use a machine specific configuration file ${Environment.MachineName}.xml for host specific settings.
  • Both files should use the same syntax and XML deserializer.
  • Settings in the host-specific file will override those in the site-generic one.
  • Configuration settings should be merged in a ValidateAndMerge method. Settings from both files will be merged, then parsed and finally validated.