Adds support for choosing a slightly larger image

When scaling images to fit a container (
img{ width: 100%; height: auto; } ), the image needs to the next larger
image breakpoint, not smaller, in order to not appear blurry. This
option fixes that.
updated minified version

compiled with uglifyjs
keep breakpoints numeric and sort correctly
Merge pull request #14 from justinph/master

Option for using next larger image at given breakpoint