Provide dc:created metadata, and provide dc:modified data for pages as well as posts

[cboettig]

• [x] Posts get dc.modified from git_modified, embedded into the sidebar.
• [x] Create a plugin to give the timestamp of the first commit on record for a given file as it's dc:created time. A bit unclear how to do this: https://github.com/schacon/ruby-git/issues/79

Is it bad form to have the dc:modified attribute provided in the site header as well? Alternatively we could create sidebars for pages (excepting index.html & lab-notebook.html, and project pages, each of which can deal with this manually), providing human-readable versions of their metadata...

[cboettig]

Posts have dc:created time on declared post date.