Use scheme-relative stylesheet URLs

The current code is problematic when code is saved locally (e.g., using http://localhost) and then pushed to a server using https. This change makes sure that the stylesheets use the same scheme as the document itself.
Added ZIP reference
Add JSON-LD LC Working Drafts
Only use scheme-relative URL if the scheme is http or https
removed empty fields
Merge pull request #196 from marcoscaceres/patch-1

Added ZIP reference
styling fix
Merge pull request #194 from lanthaler/use-scheme-relative-style-url

Use scheme-relative stylesheet URLs
Use JSON objects instead of strings and point to latest version
Merge pull request #197 from lanthaler/biblio-jsonld

Add JSON-LD LC Working Drafts
Merge branch 'develop' of github.com:darobin/respec into develop
enable escaping of nonreferences that start with [[
fix union parsing
fix test for w3c styles
silence spurious missing dfn errors
v3.1.52
Merge branch 'release/v3.1.52' into gh-pages
Merge branch 'gh-pages' of github.com:darobin/respec into gh-pages