Fix ruby version
Simplify development. Fluid mobile layout
Tweak responsive images

Always use the HiRes version and adapt their
size to the screen size
Re-add url substitution in responsive stylesheet
More generic way to have responsive images
Add Instagram icon
Fix border radius for single table mails
Increase line height
Tweak color table border colors
Lots of small tweaks
Fix responsive images with links
Define minimum width for email
Update premailer dependency
Allow to add custom string replacements in html
Add build folder task to rakefile
Tweak some styles