Update and remove broken links
Update and remove broken links

- Lecture notes using remark (http://ozan.keysan.me/ee361/) moved to http://keysan.me/ee361/
- [Markdowner](http://www.markdowner.com/) is dead. I removed the link completely, but alternatively a link to one of the following similar sites could be added [Dillinger](http://dillinger.io/), [StackEdit](https://stackedit.io/editor), [jbt.github.io/markdown-editor/](https://jbt.github.io/markdown-editor/) or [Markable.in](https://markable.in/)
- [Remarks](http://remarks.sinaapp.com/) is also dead and I couldn't find any alternative.
tripu/remark,gnab/remark,tripu/remark,FabioBeneditto/remark,FabioBeneditto/remark,gnab/remark