Warn about opening example HTML directly instead of viewing via HTTP

[cvrebert]

Viewing our example HTML files directly in the browser can break Glyphicons (see #10965) and breaks CDN links.
This commit displays an alert box when people do this to warn them about the brokenness.
/to @twbs/team for review

[mdo]

Gah, the whole reason we left the templates as whole HTML files is so folks could yank them without running a server. Sucks we have these problems, but they're not unique to our situation. Do you think this is better than just a warning in the docs somewhere?

[cvrebert]

Hmm. Now that I think about it, we can probably make the CDN links work by using HTTPS explicitly in their URLs. And only the Carousel example uses Glyphicons, so a note in that example itself would probably suffice.

That still leaves respond.js though...: http://stackoverflow.com/questions/18544310/respond-js-not-working-locally-support-for-media-queries-in-ie8