Make site.url protocol independent.

Current resources like the Bootstrap CSS are serve via http. This breaks
Browser/Extensions that deny unsecure resources in some way. Changing
the include syntax in link/img/.. tags to the protocol independent
version '//' (instead of 'http://') will always load the resources via
the same protocol as the page they are loaded from.
Merge pull request #5 from xhochy/secure-resources

Make site.url protocol independent.