TensorFlow Federated 0.5.0

Release 0.5.0

Major Features and Improvements

• Removed source level TF dependencies and switched from tensorflow to
  tf-nightly dependency.
• Add support for attr module in TFF type system.
• Introduced new tff.framework interface layer.
• New AST transformations and optimizations.
• Preserve Python container usage in tff.tf_computation.

Bug Fixes

• Updated TFF model to reflect Keras tf.keras.model.weights order.
• Keras model with multiple inputs. #416