Add s3_init rake task

As mentioned in https://github.com/imathis/octopress/pull/944#issuecomment-12956795

This adds a s3_init rake task, which will configure your S3 bucket for website hosting.

It's pretty straight-forward.  It requires aws-sdk v1.8.1, which is the first version that has the AWS::S3::Bucket#configure_website method.