add script to clean up gists

The script cleans up gists from closed/merged PR's.
This does not work, if you use ```log.error```, you get:
```
Traceback (most recent call last):
  File "clean_gists.py", line 129, in <module>
    main()
  File "clean_gists.py", line 57, in main
    log.error("Please tell me what to do?")
  File "/home/ward/opt/lib/python2.7/site-packages/easybuild_framework-1.15.0dev-py2.7.egg/easybuild/tools/build_log.py", line 100, in error
    newMsg = "EasyBuild crashed with an error %s: %s" % (self.caller_info(), msg)
  File "/home/ward/opt/lib/python2.7/site-packages/easybuild_framework-1.15.0dev-py2.7.egg/easybuild/tools/build_log.py", line 83, in caller_info
    return "(at %s:%s in %s)" % (os.path.join(*filepath_dirs), line, function_name)
TypeError: join() takes at least 1 argument (0 given)
```
And I don't understand why this happens?