update button, navbar, headings.scss files

navbar.scss:
bring @media into .navbar ruleset
add line of documentation for @media

headings.scss
add minor documentation

button.scss
add documentation
update sass from less port (consolidate rule declarations, and move the generic button mixin into the main <button>, .btn ruleset so as to cut down on the selector size when it is compiled.

Added display: inline-block to button, .btn ruleset so padding & sizing will work properly on elements that are inline by default.

Please see http://scottohara.me/min for recommendations for updated button examples