Merge pull request #22 from sbking/anti-overlap

Add anti-overlap feature w/ kwarg examples
Add retina kwarg to width mixins

Also adds `rupture.retina-queries` variable to let user define retina condition strings.
Refactor `retina` kwarg into `density` kwarg and add `density` mixin/settings
Merge pull request #25 from sbking/density-kwarg

Density kwarg
Add `landscape` and `portrait` mixins and `orientation` kwarg