you don't need unit_tests/node.sh to run a single command

The command `node unit_tests/environments/node/test.js` can be run directly from `npm test` instead of being stored in `unit_tests/node.sh`.

Now `npm test` would run on Windows as well (it no longer needs an interpreter of `.sh` files).
recommend (in README) the `npm test` command to run unit tests

That's more cross-platform (would run on Windows) than shell scripts.
delete unit_tests/node.sh

The file is no longer used in `package.json` and `README.md`.
Merge pull request #344 from Mithgol/windows-testing

enable testing on Windows