session_save_hist_index: option to save buffer last-access order in sessions
CREDITS: update
session.js: whitespace, style
session_save_hist_index renamed to session_save_buffer_access_order

'hist_index' property within sessions renamed to 'access_index'
youtube-player.js: update play/pause and fullscreen

The HTML5 play/pause and fullscreen buttons are no longer button
elements, but div elements with a .ytp-button-* class.