Fixed the handling of integer data types in the raw ops generation script. (#273)

* Makes 'Tensor.gathering' generic over 'Int32' and 'Int64'. Fixes #259.

* Added a 'TensorFlowIndex' protocol.

* Better handling of integer types in the raw ops generation script.

* Rename.

* Merge conflict fix.

* Reverted part of the changes.

* Removed the 'TensorFlowUnsignedInteger' protocol.

* Reverted a change.

* Reverted another change.

Author: eaplatanios