Code review comment for lp:~jkakar/bzr/custom-project-name

Thanks a lot for the feedback and sorry for taking a while to
respond. I've made the following changes to the branch:

- bzrlib.application_name defaults to 'bzr' and can be overridden by
  programs using bzrlib.

- Variable substitution logic for help topics has been removed.

- bzrlib.application_name is used by bzrlib.commands when generating
  help output.

I thought about adding application_version as suggested, but I
decided to leave it out for the time being since I wasn't changing
anything that would use it and version_string is already present.