Add other systems integrating with remark to README.
Change script tags hotlinking remark from using .com to .io in src attribute.

E.g. chrome displays insecure content warning and blocks loading remark when the slideshow is served from another gh-pages page on github, as github has started redirecting urls from `somepage.github.com` to `somepage.github.io`.
Merge pull request #45 from torgeir/patch-1

Change src for script tags hotlinking remark from using .com to .io
Merge changes from remote.
Use .io instead of .com.
Fix issue #47 by extending HTMLCollection with forEach function.
Merge changes from develop branch.