Feature/product review prebuilt model [resolves #221]

[irmakyucel]

• Update previous PR for compatibility with latest developf6f054d.
• Create product_sentiment.py, test_product_sentiment.py
• Optimize and build the product review sentiment model
• Change README file accordingly
• Change product_sentiment.py for working with ICU tokenizer
• Change test_product_sentiment.py due to the changed class labels
• Add product review sentiment model to context.py file
• TODO look for ways to improve this score. Report @dafajon with possible replacement for dataset.

[husnusensoy]

@irmakyucel Please try to improve model accuracy and resend the pull request. Also add any relevant work with the dataset.