Added custom path for the kaggle credentials file.

[luiscberrocal]

Added a custom credentials file for the function opendatasets.utils.kaggle_api.read_kaggle_creds. This allows the flexibilit of not having it on the local directory. Added tests for the changes and requirements_dev.txt for development requirements. Added Makefile to run tests by running make unittest Did some linting with flake8 and mypy.