Added code to keras implementation

[LucasKirsten]

The two files contain code for implementation in keras to the PIE network.

The file on the model folder contain a class PieAPP that builds the model, load weights from the tensorflow checkpoint and predicts the pie value for two images for a choosed stride. In future implementations, I intend to add code for training and loading/saving the model directly from .h5 files like is commonly done in keras (still have some bugs doing this last).

The test file works just like the test files from tensorflow and pytorch implementations.

This implementation will allow other researches to explore the PIE with the keras framework.

[prashnani]

Thanks @LucasKirsten ! Apologies for the delayed response. Merging after testing.