Text input based models should optionally not parse examples

[sorensenjs]

For simplicity, instead of taking a TF Example with a single text feature, we should just input the tensor directly. This means the tf hub and char models should use

tf.estimator.export.build_raw_serving_input_receiver_fn

in place of the

build_parsing_serving_input_receiver_fn

which would eliminate at least one of the blocking ops for tensorflowjs. #222