updates to flair methods for praw4
updates to flair methods for praw4

- delete_flair is non-existent, replaced with update() with no flair arguments
- get_flair_lsit is non-existent, replaced with SubredditFlair attribute
- flair list now contains Redditor class instances, not just usernames, so sorting needs to use the name attribute

refs praw-dev/prawtools#40

praw-dev/prawtools