Add Keras Counterparts

[j-planet]

The Keras counterpart resembles Andrej Karpathy's original PyTorch code as closely as possible. Not only was this good practice for myself, I hope this will help the Keras practioners as well as broaden the readership of this repo.

Edit: Fully tested. Works out-of-the box by running keras/gpt.py. more.txt is a sample output.