Improvements to support for BitBurner boards

--bitburner-fury-options allows avalon-options to be overridden for
BitBurner Fury Boards, facilitating simultanous use of BitBurner XX
and BitBurner Fury boards

More sensible defaults for BitBurner boards, so cgminer should do something
sensible without any command line args
Code cleanup and improved documentation
Remove modified Bitburner avalon defaults - I was mistaken about maximum
stack size
Merge pull request #509 from roybadami/master

Improvements to support for BitBurner boards