Installation Clarifications - Updated to Include Recursive Downloads

I have a few questions related to installation of this plugin/task, so I'm submitting with a pull-request with changes to the documentation for how I was able to get this working.  _Preface: I don't really do much with npm and this is my first week with grunt._

Some questions that possibly some other folks new to grunt may be asking:
* The docs indicate that this is loaded via loadNpmTask, but running `npm install grunt-s3 --save-dev` doesn't create the `tasks/lib/common.js` or `tasks/lib/s3.js`.
* Am I missing something that would install the npm dependencies? The grunt 0.4.0 installation of this plugin never fired those dependencies, so I was firing them off manually as well. Is it normal behavior that `npm install grunt-s3 --save-dev` _wouldn't_ install those dependencies?

This may just be a noob issue, but after reading grunt docs on the difference between `loadTask` and `loadNpmTask`, it seems like this acts more like `loadTask` with its required `lib` assets, without a built-in npm mechanism for installing those.

Feel free to reject this Pull Request, just wanted to submit something of use if I'm on the right track here.