Switch to redcarpet 2.2 for markdown processing

- redcarpet needed by the picturefill plugin. Note that v2.2 used here
  doesn't support footnotes, but the jekyll HEAD will support v3, so
  look to upgrade soon.
- Also added minimagick gem to the Jekyll gemfile in prep for using
  picturefill.
Use jekyll-picture-tag for reponsive images.

- The full-sized images are kept in assets/images/_fullsize.
- Settings picturefill media queries in in _config.yml.
- matchmediapicturefill is a compressed version of
  - https://github.com/paulirish/matchMedia.js/blob/master/matchMedia.js
  - https://github.com/scottjehl/picturefill/blob/master/picturefill.js
- I had ot use an old version of the picture_tag.rb plugin, see
https://github.com/paulirish/matchMedia.js/blob/master/matchMedia.js
- Try out picturefill on fieldnotes post