Twitter/Open Graph description meta fields

Addresses #38 

Rather than injecting all of the content of a page/post, which winds up as un-rendered Jekyll expressions/loops I introduced a `description` variable to each post's front matter.  The text was pulled from the header for each page, so it should look familiar.

I was going to manually do the same for each post, but Jekyll is smart enough to grab the first `<p></p>` tag and use that as the description.  If the first paragraph is rather short it uses the first two paragraphs.  If the paragraph is particularly long it truncates the field and adds ellipses.

With this PR when you post a link to a maptime page on Facebook or Twitter it should include the description automatically.