icons, trending subs and sub interest bar

- set vote arrow alpha in the image directly
- new icons for trending subreddit, private subreddit, over 18
  subreddit and cakeday
- rename promoted link style to %tn-special-entry and apply it to
  promoted links, new/upcoming links in front page, and subreddit
  interest bar (the subscribe to new subreddits input with upside down
  snoo)
- force custom logo of subreddit to not exceed $page-title-height
- add back -webkit-linear-gradient to linear-gradient mixins
whoops, forgot that scss/userstyle/_trend.scss is removed

its contents have been moved to scss/userstyle/_list.scss
license and miscellaneous fixes

- added license
- replaced header banner by a better photograph that allows derivative
  work
- use jpegs for noise bg and make them smaller
- move infobar style as placeholder selector %tn-info-box
- added style for highlighted single comment thread
- force our visited link color when RES marked a media link as "visited"
- force subreddit logo width to 300px. some subreddits uploads their
  banner as sr logo.
- use custom triangle for next/prev in organic-listing instead of using
  unicode char. for some reason with unicode chars, if i make them aligned
  in firefox, they will be misaligned in chrome. and if i align them in
  chrome, they'll be misaligned in firefox.
- smaller buttons in subscription box (in multireddits)
- few fixes in profile page
- updated readme