Completed all required methods for Playlist/PlaylistRepo class.

-Playlist now initializes with the properties id, name, songs, and description.
-PlaylistRepo added functionalities:
1. create table playlists in songify-db
2. drop table (for testing purposes only)
3. add (takes in params from the post request in psql)
4. finds all playlists in database and returns it as an array containing instances of Playlist class.
5. builds params into an instance of the Playlist class.
-All tests passed for both classes.