nav & selection updates

reduce the length of selectors for the nav partial

also move unprefixed ::selection style declaration after the prefixed versions, created a new mixin for it so the styling could be updated in a single place.
remove -webkit-::selection

there is no -webkit-::selection, it uses webkit browsers use ::selection by default.
Merge pull request #53 from scottaohara/nav_selector_minification

Nav selector minification & ::selection edits