COPYING: Add MIT license for this project

The text is from http://opensource.org/licenses/mit-license.html.  I
haven't made any local changes yet, so the `<year>` and `<copyright
holders>` markers are still there.

Because I'm not sure I want to use this license yet (maybe I'll use
the BSD license instead?), I've started a new branch for this commit:

  $ git checkout -b mit-license master

That creates a new branch `mit-license` based on the current `master`
and checks out the new branch.  You can see which branch you're
usually on using `git branch`:

  $ git branch
  * mit-license
    master

The asterix marks the active branch.  Once we've checked out the new
branch, creating commits is just like it used to be:

  $ git add COPYING
  $ git commit

Projects usually include licensing information like this explaining
the terms under which others are allowed to make changes.  The file is
often called COPYING or LICENSE.  The Open Source Initiative has a
list of popular licenses [1], as does the Free Software Foundation
[2].

[1]: http://opensource.org/licenses/
[2]: http://www.gnu.org/licenses/license-list.html