Only create a Checkable for a file if it is to be added to the sample.
 - Update CrawlConfig.read() to handle 'include' in config files.
   Added a test for this feature.
 - Move method write_cfg_file() from crawl.py and CrawlConfig.py to
   testhelp.py so one copy is shared rather than having two copies
 - Separate CrawlConfig.py and CrawlConfigTest.py to facilitate moving
   write_cfg_file() into testhelp, which required importing
   CrawlConfig into testhelp
 - Updated all_tests.py to look to CrawlConfigTest.py rather than
   CrawlConfig.py
Updated Checkable.check() to handle HPSS being unavailable gracefully.