Add automated deployment.

This looks for the following environment variables for deployment:

* `AWS_ACCESS_KEY_ID`
* `AWS_SECRET_ACCESS_KEY`
* `AWS_BUCKET`

You can add these as secure environment variables via the following commands:

```sh
gem install travis

travis encrypt AWS_SECRET_ACCESS_KEY=PRIVATE_DATA_HERE --add env.global
travis encrypt AWS_ACCESS_KEY_ID=PRIVATE_DATA_HERE --add env.global
travis encrypt AWS_BUCKET=PRIVATE_DATA_HERE --add env.global
```

It is possible to do this via Travis's `S3` deployment target (see [here](http://about.travis-ci.org/docs/user/deployment/s3/)), but I find it better to be able to do deploys locally also.

Note that this assumes that the site is deployed to the root of the bucket, if that is not the case you will need to tweak the settings in `tasks/options/s3.js`.