Minor updates to README and docs

- Updated Skin guide to reflect changes since 4.0
- Fixed a few broken links (and made them relative)
- Updated Setup guide to 4.1
- Fixed HTML5 Boilerplate references in examples to say Video.js

To make sure some of these relative links worked I needed to push to my repository, so that made an interactive rebase a little difficult. ff59c98 was supposed to squash the last 4 commits, but because remote and origin had diverged it just made things worse. If it's a big deal I can just move everything to a new branch so it's all wrapped up in one commit.