Check in package.json file; remove Makefile.

- Make isn't super cross-platform / easily available on e.g. Windows.

- `make test` was always returning a successful exit code, even if there were
  failures. That was fine during development, but not for tools like Travis.

- We could alternately have `make test` fail on test failures, but then it
  wouldn't get a chance to `clean` -- remove the compiled package.json file.

That last one may have been solvable, but ultimately, it's probably better to
just check in the package.json file.