Add support for attachments

[ThomasKranitsas]

• Add a new model for attachments.

{
  id: "UUID",
  url: "String",
  fileSize: "Number",
  "fileName": "String"
}

• Implement an endpoint to upload attachments on an S3 bucket.
• Add an attachments properties to the challenge model that will be an array of Attachments.
• Update all challenge endpoints to support attachments.

[ThomasKranitsas]

Challenge: http://topcoder.com/challenges/30089434