move from downloads.html to customize.html to better match purpose of page, fix up customize page layout to match other docs, link style refinement to jumbotrons

remove popoverarrows mixin, relegate code to tooltips and popvers since we only used them once each; this fixes the problem of broken tooltips in 2.1

adding getting started page based on Bootstrap University from awhile back, testing out side nav on docs instead of subnav bar

use variablef or wells background-color, update docs for glyphicons license mention, update popovers to remove that thick border and make them look mo betta