Closes #13: Add dependencies to requirements.txt and create .gitignore

Improvements

• I completed the incomplete requirements.txt file, such that the dependencies may be installed by execitong

  pip install -r requirements.txt

• I created the .gitignore file and removed pollutants from commit cache. I also made sure the flask_session directory is allowed to persist by adding a flask_session/.empty file to the commit and making sure .gitignore doesn't exclude it.'

Tasks

• Complete requirements.txt.
• Add .gitignore.
• Remove pollutants by deleting them from the new commit (git rm --cached <...>).