Add responsive images, closes #17

Responsive images work. All images in img/responsive are resized
automatically and picturefill is loaded by default. The referencing
has to be done manually, which is a chore, but right now it's the most
simple solution.

- updated grunt-modernizr config to always load the html5shiv
- an image partial that also takes parameters wasn't possible (it doesn't
  work with page nesting and the assets tag)
- added picturefill with bower and added it in the head
- started using the {{assets}} tag so assets are always correctly referenced
Merge branch 'picturefill' into develop