0.3.0

• Add a data_file resource
• Add a signed_url resource
• BC break replace dataset.data_file and source.data_file objects by dataset.data_file_id and source.data_file_id reference:

Instead of

{
  "dataset": {
    "..."
    "data_file": {
      "id": "54edf76c6170700001870000",
      "filename": "hello.csv",
      "type": "S3",
      "size": 24,
      "url": "http://prod.kml-api.s3-us-west-2.amazonaws.com/uploads/5347b31750432d45a5020000/sources/1415101671848/hello.csv"
    }
    "..."
  }
 }

... we now have:

{
  "dataset": {
    "..."
    "data_file": "54edf76c6170700001870000",
    "..."
  }
 }

{
  "data_file": {
    "id": "54edf76c6170700001870000",
    "filename": "hello.csv",
    "type": "S3",
    "size": 24,
    "signed_url": {
      "links": {
        "self": "https://api.predicsis.com/data_files/53c7e7b668796493d3010000/signed_url"
      }
    },
    "fileable_type": "Dataset",
    "fileable_id": "5399a07170632d4b0c010000"
  }
}

{
  "data_file": {
    "signed_url": "http://prod.kml-api.s3-us-west-2.amazonaws.com/uploads/5347b31750432d45a5020000/sources/1415101671848/hello.csv"
  }
}