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 goes to github users @Flaeme and @danny316p for the fix.
Have a better error message when dealing with non-ghar symlinks.
Also fix path discovery for symlinks.

Use realpath instead of abspath so that ghar can find its directory even
if "ghar" is a symlink to the script, as well as the case where it's
invoked with a relative path. Also reformat line in accordance with
PEP8.
Merge branch 'fix-path-discovery'
Merge remote-tracking branch 'jacobian/symlink-error-message'
Fix typo in symlink error message.