Support string as an input type

[thunterdb]

The tf.string type is required to properly load images into a graph, without having to first go through python. This one may be a bit more tricky than the other types since it is an array type.

[thunterdb]

The java example may help in that respect:
https://github.com/tensorflow/tensorflow/blob/master/tensorflow/java/src/main/java/org/tensorflow/examples/LabelImage.java