added last modified plugin

added so that we could compute the true last modified date for the site
map

https://github.com/nquinlan/jekyll-git-last-modified

named “aaa_last_modified.rb” to guarantee it’s loaded first in case we
ever want to have other things use this plugin (and of course jekyll
has no dependency structure or anything)
modified site map generator for correct dates

this makes all the dates on the sitemap correct to the last modified
date
corrected file paths

removed a problematic dir

further allowed for an index.html as that’s the canonical link for us
added a check for date, before assuming it existed

because everyone knows what mary poppins says about assume
modified documentation
Merge pull request #417 from sendgrid/new-sitemapper

improved sitemapper