added ann capability endpoint to predict scoring

[Tee-Hui]

Created endpoint for score prediction with keras model saved in h5 format.

[ashishawasthi]

• requirements.txt should have keras as a dependency
• PredictionError text should be designed for API client

[ashishawasthi]

I think PredictionError should be:
Unknown model_lib {self.model_lib}
as the caller of API may not have access to the code.

Name of second model should be iris_ann_v1 to follow the naming convention of first one.

[ashishawasthi]

there should be test cases added to test_app.py for ANN, the way test cases are covering the sklearn model