Add support for deleting from playlists and some improvements to how data is sent with a request

This PR has a few things beyond the two issues mentioned in the commits (#21 & #22):
 * Add a requirements.txt
 * Some PEP8 clean up.
 * Handle request data for POST/PUT/DELETE more generally 
 * Rely on request's raise_for_status to identify issues with a request. 
 * Report send specific error messages to the SpotifyException, taken from the response json
 * Tweak some documentation