Change slides link from HTTPS to HTTP

For some reason the fonts are messed up when accessing with HTTPS.
Changes .gitmodules from git to https

Makes it less confusing to explain the diff between
master and gh-pages.
Merge branch 'master' into gh-pages

Conflicts:
	index.html
Use goo.gl shortcut link for QR code
Merge branch 'master' into gh-pages

Conflicts:
	index.html
Add Google Analytics tracking cookie