Use redcarpet instead of maruku to render markdown.
Add dependencies and code for jekyll-picture-tag plugin.

* jekyll-picture-tag: https://github.com/robwierzbowski/jekyll-picture-tag
* mini_magick gem: https://github.com/minimagick/minimagick
* picturefill.js: https://github.com/scottjehl/picturefill
* matchMedia.js: https://github.com/paulirish/matchMedia.js
Add configuration for jekyll-picture-tag and include required JS in layouts.