Improve package documentation, add fetch(), and add conn file usage

### Overview of changes:
* Add documentation related to setting the path in README.md per #5
* Add function fetch() per #9. This implementation simply downloads the files as csv files and loads them into a list()
* Add usage of a conn file for transferring database connection details (e.g., user) to the function install_data() when working with mysql or postgre databases, per #7. This is simply a first pass at a solution to the problem of a need to securely transfer passwords.