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)`.

### Benefit

No need to remember to run `npm install` or `git submodule ...` when a new component submodule is added or `package.json` is updated. `make` file dependency tracking takes care of this.

### Acceptance Criteria

```
% cd /tmp
% rm -rf brick ; git clone https://github.com/mozilla/brick.git ; cd brick
% make demos # notice the error
% curl -s https://raw.github.com/wilmoore/brick/quick-contributor-start/Makefile > Makefile
% make demos # this time all the dependencies will be worked out and the demos will be created
% make demos # all up-to-date tasks won't run again this time...the demos will be made either way since it sits in a `.PHONE` target
```

The above steps should result in updated submodules, npm install, and demos being created without errors.