simplify testing

This pull request contains two changes:

1. It's possible to run `npm scriptname` without prepending `./node_modules/.bin/` to the names of locally installed Node modules' CLI commands. I've removed `./node_modules/.bin/` from `mocha` and `coffee`.

2. The repository does not contain a makefile, and thus `npm test` becomes more reliable than `make test` for testing. It's also shorter to type, and thus I've given `npm test` in the corresponding example (in `README.md`).