WORK IN PROGRESS: Adds creation of ./built_dotfiles/installed_build symlink.

The target for the "installed_build" link is chosen either by:
    - referring to the target of a "latest_build" link in the same "built_dotfiles" directory, or
    - if such a symlink can't be found, sorting all sub-directories at "built_dotfiles/built_at_*" by name and choosing the last/highest one. Since these directories are normally named with a sort-friendly timestamp (eg "built_at_2014-04-25_20-45") this should choose the most recent build.

Next steps are adding a symlink from ~/.dither_dotfiles to this "installed_build" symlink, and creating symlinks via ~/.dither_dotfiles to each dotfile in installed_build/ (eg from `~/.mydotfile` to `~/.dither_dotfiles/.mydotfile`.

At each of the above steps we should also politely move aside / backup any files we might otherwise be destroying. For example, if `~/.dither_dotfiles` already exists and points to somewhere other than our `installed_build` symlink, we should move it to `~/.dither_dotfiles.moved_by_dither_at_<current time>` and leave its contents intact.

Similarly, if our build contains `.mydotfile`, and there is already a `~/.mydotfile` file which isn't a symlink to `~/.dither_dotfiles/.mydotfile`, we should move that file to `~/.mydotfile.moved_by_dither_at_<current_time>`. Remember the existing `~/.mydotfile` could be a symlink to elsewhere, a file, or a directory (eg `~/.ssh/`); all of these cases should be handled appropriately.