Adds first work at dither, with build support.

The build command is based on an older build.py script from my dotfiles repo (it was called "dotfiles_jinja", but this is likely to have changed since writing).

This is very rough and not ready for use yet.  This was mostly committed to get the work off my laptop, but I hope you'll forgive that of the first commit. :)
Adds initial work on link command

Also includes planning notes for link command, which may be removed later.
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.
`dither link` now links ~/.dither_dotfiles to installed_build

dither will first check if ~/.dither_dotfiles exists. If it does exist but doesn't point to installed_build, dither will politely move it aside to a filename like ~/.dither_dotfiles.moved_by_dither_at_<timestamp>.

Next steps: see previous commit for more detail, but generally, making links in ~ via ~/.dither_dotfiles for each of the built dotfiles. eg Linking ~/.bashrc -> ~/.dither_dotfiles/.bashrc, if our build contains .bashrc
dither link now creates per-file links in home directory.

eg if the build output directory contains .bashrc and .Xinitrc:
 - `dither link` first creates ~/.dither_dotfiles linking to the build output directory
 - `dither link` then creates ~/.bashrc -> ~/.dither_dotfiles/.bashrc and ~/.xinitrc -> ~/.dither_dotfiles/.xinitrc

Note if ~/.bashrc already exists and doesn't point to ~/.dither_dotfiles/.bashrc, `dither link` will move the existing ~/.bashrc to ~/.bashrc.moved_by_dither_at_<timestamp>.
Adds note of possible bug in `dither link`
`dither build` now creates "latest_build" symlink after building.
Adds `dither update` to run `dither build` and `dither update` in sequence.
Fixes bug in path used for "latest_build" symlink