Multi-directory support for grocery_delivery

Also update the docs for both this and multi-VCS support
Merge pull request #4 from jaymzh/multidir

Multi-directory support for grocery_delivery
Enable people to override the underlying VCS from hooks

This will allow people to set their VCS so GD can choose the right
module to load, but still override the underlying implementation, ala:

  VCS=/usr/bin/git

  my_git_wrapper() {
    ...
  }
  GIT=my_git_wrapper

And same for $SVN.

Which was possible in the old implementation but got dropped in multi-VCS
support. This re-adds it.
Update docs for VCS handling

Document the new variables as well as generally fix documentation on multi-VCS
support.