Provide `grunt npm:test` for verifying NPM package functionality

This basically calls `npm pack`, installs the resulting package in a temporary directory, then requires it and attempts to use the `.transform` method. Bonus: `bin/jsx main.js`.

The `npm pack .` command takes a long time on my machine. When I run it manually with `--verbose`, it seems to be hanging on the `npm verb lock` line, so it seems to be an `npm` problem.

Closes #12.