Merkyl update

Request to be able to add log files to merkyl on the fly and then
retrive and search their contents. To do this the following was
implemented.
* Added MerkylInspector class with the following methods
 * add_log() - adds a log file to the current merkyl instance
 * get_log() - gets the contents of a previously added log
 * search_log() - convenience function doing the get_log() and a simple
   search in one go.
* Added merkyl_inspector fixture which returns a MerkylInspector for the
  current appliance.