COPYING: Add the 2-clause BSD license for this project

COPYING: Add the 2-clause BSD license for this project

The text is from http://opensource.org/licenses/BSD-2-Clause.  I
haven't made any local changes to the text yet, so the `<YEAR>` and
`<OWNER>` markers are still there.  I did wrap the original text to 70
columns to it's easier to read in narrow terminals (which are
traditionally 80 characters wide).

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

  $ git checkout -b bsd-license master

That creates a new branch `bsd-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
  * bsd-license
    master
    mit-license

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].

The contents of this commit message are very similar to e8051dc
(COPYING: Add MIT license for this project, 2013-11-29), but I don't
expect both commits to be merged into the master branch.  It's better
to have a commit message that makes sense even for folks that don't
have access to e8051dc in their repository.

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