Staging deploy task

Adds a new Grunt task `grunt staging-deploy`.

When you run the task, it builds the site and deploys the `dist` directory to an S3 bucket into a folder in the root with the same name as your branch.

For example, the staging URL for this branch is: http://optimizely-marketing-site-staging.s3-website-us-east-1.amazonaws.com/krush-staging-deploy-dev/

This also gets rid of the `grunt preview` task. And additionally, changes the `configs/sample_s3Config.json` to remove the `bucket` property and adds a `staging_bucket` property.

If you want to test this, break something on the homepage, run `grunt staging-deploy` and then visit the URL above and you'll see your change.