Verify binaries while creating release blog post

[phillipj]

At the moment of creating the release blog post, there's a chance not all binaries has been uploaded to nodejs.org/dist/ yet.

In these circumstances those specific binaries should have "Coming soon" rather than the actual download URL as those URLs would fail with a 404 page anyway.

This change enables that behaviour by verifying all the binaries about to be included in the blog post, by doing a HEAD request against each of the URLs where they're supposed to be located.

Refs #156

cc @nodejs/release

[rvagg]

Merged and I used this for v4.1.1. I had to fix the ARMv7 link to make it armv7l (missing l) but it worked aside from that.

I did a couple of extra things for this release that are not covered by the script:

• removed the ## header of the release, I don't think we need it given that all of that information is contained in the metadata of the post
• added author: Rod Vagg, it would be nice to automate this, we could either use the GitHub API or just web scrape to look up the release tag (v4.1.1) and find the commit for that tag and look at the author behind the tag (you can get the git.name which is probably the best one to use here). Another alternative is to scrape the README @ https://github.com/nodejs/node/blob/v4.1.1/README.md#current-project-team-members and look at the username / name mapping and match that against the username included in the CHANGELOG.md ## heading. Or .. just hardwire a mapping in the script here might be easiest!
• added a bit of summary text at the top, but this doesn't need to be automated and often won't be there

great work on this though!

[phillipj]

Thanks! I'll add two tasks for the improvements you mentioned.