Add a Cake task for syncing assets to S3

The task is called `cake cut` as in "cut a release" and whathaveyou. :cake:

- Requires the node package `s3` to be installed
- Requires you to set some environment variables for S3

I've updated the README accordingly.

The task currently assumes you'd like to upload `lib/boomerang.css` and `lib/boomerang.js`, but it could be made to accept arguments in the form of filenames or probably a pattern.