added filter to overwrite e-mail requirement when e-mail field is removed in settings

issue #25
renamed and moved filter_ignore_comments_open
fixed require_name_email filter to only overwrite option on guestbook pages
added option to overwrite registration requirements for comments on guestbook page

option name: cgb_ignore_comment_registration
issue #26
Added and implemented option "cgb_threaded_gb_comments"

issue #24
css improvement for radio setting values
updated screenshots
preparation for version 0.6.6
Merge branch 'develop'