v0.4.0 - Upgrade Transformers Version

• Updated the dependencies so that it works with Huggingface Transformers version 4. Sentence-Transformers still works with huggingface transformers version 3, but an update to version 4 of transformers is recommended. Future changes might break with transformers version 3.
• New naming of pre-trained models. Models will be named: {task}-{transformer_model}. So 'bert-base-nli-stsb-mean-tokens' becomes 'stsb-bert-base'. Models will still be available under their old names, but newer models will follow the updated naming scheme.
• New application example for information retrieval and question answering retrieval. Together with respective pre-trained models