Responsive Images!

GRUNT
- Add a responsive images task using grunt-responsive-images.
- Avoided implementing into build task due to it having a plugin
reliance one must install on their machines to create graphic sizes
- Plus this is really only helpful for non cms editable images

JADE
- Added a picture and srcset jade mixin for easy implementation with the
new grunt task!

BOWER
- Added a polyfill to allow for srcset and picture elements work on
older browsers

TO DO
- Add new jade mixin for srcset using 'x' instead of width, useful for
high res screens
- Inspect whether Modernizr is needed due to lack of IE8 support and if
so is it needed in the header and can be moved to the footer?
- Inspect whether Repond.js is needed due to lack of IE8 support
- Additional research on critical css and automation into build task
- Research on lazyloading images to further reduce requests