makefile: integrate xyz

Closes #83

This pull request integrates [xyz][1] to further automate the release process. It also introduces a prepublish script (run by xyz, not npm) which generates the JavaScript files. This means it's no longer necessary to include the JavaScript files when committing changes. Thanks to `git update-index --assume-unchanged` (which is run as part of `make setup`) changes to JavaScript files do not appear in the output of `git status`.


[1]: https://github.com/davidchambers/xyz