datascienceinc/Skater: Minor Improvements

• Simplified Installation to handle dependencies better
• Convenience functions to support flipping image orientation flip_orientation
• Updated examples on how to use skater with xgboost
• Enabled support for Image Inference using Occlusion with the ability to specify window_size and step_size for perturbing feature space
• Other minor bug fixes and code clean-ups

Credits: To all contributors who have helped in moving the project forward.