Fix issue with getting ghar_dir when sys.argv[0] is a relative path.

Fix issue when ghar invoked with relative path ('bin/ghar' or 'ghar/bin/ghar')
or when 'ghar' invoked in ghar/ dir and relative path 'bin' is in the PATH
(e.g. when using Boxen).

Credit github users @Flaeme and @danny316p with the fix.