Depend on update_checker >= 0.6. Obviates #239.
Add method select_flair
Add errors BadCSS and BadCSSName
Make test_delete_invalid_image use BadCSSName

A upstream fix (see issue 883 on Reddit's github page)
made Reddit properly throw a BadCSSName instead of a 500
status code on using invalid image name in
Subreddit.delete_image.