v0.3.0: Change CNN encodings, drop python 3.6/3.7 support

Installation fix

• Make package installable by removing tensorflow as a dependency and replacing it with pytorch #173
• Drop support for python 3.6 and python 3.7 #173

✨ New features and improvements

• Use MobileNetv3 for generating CNN encodings #173
• Introduce a 'recursive' option to generate encodings for images organized in a nested directory structure #104

Breaking changes

• Size of CNN encodings is 576 instead of 1024 #173
• Since CNN encodings are generated using a different network, the robustness might be different; user might need to change similarity threshold settings #173
• Hashes (all types) may be different from previous versions for a given image #173