Credit website authors in an auto-generated AUTHORS file.

The initial version of this pull request is a work in progress.  Don't
merge yet!

Motivation: as I mentioned in #63, I've written an
`update-copyright.py` script [1] to automatically keep track of boring
copyright/license details.  The script reads the VCS history to
generate a list of authors who have contributed to the repository (it
can also automatically update per-file copyright blurbs, but I'm not
using that here yet).  Because SWC doesn't require copyright
assignment, something along these lines (even if it's just maintaining
the `AUTHORS` file by had) is probably required, or we'll be violating
the MIT/CC BY license under which we accept patches from contributors.

Drawbacks to using my script over updating `AUTHORS` by hand:

* Another dependency!  Although if update-copyright becomes to
  burdensome it is easy to switch to manual maintenance later.  Just
  remove `.update-copyright.conf` from version control.

Benefits to using my script:

* You don't have to remember to bump the `AUTHORS` file ever again.
  Just run `update-copyright.py` before tagging a release (or once
  every few months).

Issues to be resolved:

* I don't have names for a goodly number of
  http://svn.software-carpentry.org contributors, and I don't have any
  information at all about where the pre-2009 content came from (it
  was in http://swc.scipy.org/svn/All, but then lost during the move
  to http://svn.software-carpentry.org).

This PR, if accepted, should probably be rebased on top of #177 to
avoid double-counting contributors with inconsistent Git author IDs.

[1]: http://pypi.python.org/pypi/update-copyright/
     http://blog.tremily.us/posts/update-copyright/