added path_key to include read path into the event under this key

When sending the events to the elasticsearch it''s quite useful to be able to understand what file is the origin of the received event. 
The patch adds additional optional 'path_key' tag to the configuration, to hold the filename

    <source>
      type tail
      path_key path
      path /var/log/syslog
      tag syslog
    </source>

All the changes are backward compatible as the path is inserted into the event only if path_key is specified.