Add mdk run option to to list what scripts are available

This is wrong on so many levels:
* The business logic is in the moodle class, but the logic is not
  related to a Moodle instance
* The argument to list scripts should be added as a proper one, not overided
  the value of 'script'
* The code which gets the list of directories structure should be
  abstracted so it can be used by listScripts and runScript rathe than
  duplicated
* We probably should list the path as well as the script name
* This is some of my first python code, so I don't even know if i'm
  using the right methods

So whatever you do, DON'T merge this!