Enhances `Makefile` to make build dependencies self documenting.

### Summary

- All build tasks requiring `npm` dependencies depend on `node_modules`
  target.
- `node_modules` file/directory target depends on `package.json` and
  `dist/*/*.min.js` files. Person building can never forget a step.
- Use the variable `$(RM)`.

### Acceptance Criteria

```
% cd /tmp
% wget https://raw.github.com/wilmoore/brick/quick-contributor-start/Makefile
% make demos
```

The above steps should result in updated submodules, npm install, and demos being created without errors.