Show summary and description; always Markdown

[rking]

As a new gem author, I was pretty puzzled by the contrast between:

• The specification docs, which say: "The description should be more detailed than the summary." ( http://guides.rubygems.org/specification-reference/#summary
• The actual output on the site, which only shows the summary if both are present.

Additionally, it seems like many people try to put line breaks in these fields, but the site just puts them all in a <p> unless the text happens to match the very specific regex: /^==/

So this patch should rectify both. I hope you like it, but if not, let's talk about the options. Thanks!

[rking]

Related To

#377 and #395, which do not seem to be working at the moment: https://rubygems.org/gems/minitest

Build Failure

Has been going on for a few commits: https://travis-ci.org/rubygems/rubygems.org/builds

I can debug that later, if you want. But this patch has tests and they pass.

Legacy Issue

Some gem authors are in the habit of redundantly stating the summary in the description, probably due to the display logic of the existing rubygems.org

If you guys want, we could description.gsub summary, '' It's a patch solution, but it could smooth the adoption of this new look.

[qrush]

I don't want to encourage any use of markdown inside of the gemspec. Just simple plaintext is supported, and then your README on github, docs, etc can be better formatted.