Initial commit of the fitshead script

Improved the help text

Improved the help messages and PEP8 compliance

Avoid crashing on a negative HDU extension number

Fixes Python 3 compatibility issue
Added support for EXTNAME/EXTVER HDU accessing and JSON output

removed JSON support; removed os.linesep; tolerate broken pipe errors
removed JSON support; removed os.linesep; tolerate broken pipe errors