Amazon S3 as an artifact deployment source

I put together support to pull deployment artifacts from S3 in addition to Nexus, local files and URLs.  This is based entirely on the url provided - S3 artifacts must be in the form: ```s3://bucket-name/path/to/object```

Included in this is a change to the format of the data_bags - I made it backwards compatible to current and previous formats, and wrote rspec tests around it as well.

I am fairly new to Ruby (and never done rspec tests before) - so suggestions or comments are welcome!