utils: Add to_char_or_null helper
Convert all lambdas to functions
utils: Add TODO on the tight sleep loop in load()
tests: Extract code repeated four times so far
playlist: Clearify initially_unload_playlists/in_ram relationship
docs: Diff between pyspotify 1.x and 2.x; split out dev setup
docs: Add section on properties vs getters/setters
Revert "session: Rename playlist{_container => s}"

This reverts commit cf838c6d32b592293a2897594eb99d55fdc9f331.
utils: Add docstrings to all utils