Remove commented line.
Refactor walker, add tests, fix bugs.
Test explicitly for non-first author line causing exception in walker.
Do some minor cleanup.
Don't attempt to jump to /dev/null as a legit version of a file.
Add next / previous and beginning / end thread navigation in emacs.

Also, alter the keybindings to better fit the emacs convention of f /
b as "small" units and n / p as "larger" units.
Re-center buffer after {next,previous} {comment,thread} in emacs.

Per Dan's excellent suggestion.
Add simple mailboxing.
Add diffscuss-mode keybindings for mailbox integration.

Involves adding options to the mailbox commands to print information
useful to the emacs mode.
Remove check keybinding in emacs mode.

It will just be confusing, since it's only available on diffscuss
files.

Better to alert people to the diffscuss-mb-check function, and let
them bind it globally if and as they choose.
Make new lines work in emacs when cursor is in leader.

Closes #21.
Add a first cut at a Vim plugin.
Set unified lines to 20 in gen-diffscuss.py.

To make it easier to see context in reviews.