update Extend docs page to include side nav and spec out next section for extending with static assets

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

fixes #3291: remove float on responsive .thumbnails for 767px and down to ensure thumbnail images don't pop out of container in FF

fixes #3210: add .btn to .navbar-form to remove bottom margin, improve vertical align of contents of .navbar-form

Update linear-gradient standard

Conflicts:
	less/buttons.less

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

Full height progress bar

Adding 'connect' npm requirement as it is required to run `make test` (in order for it to run `node js/tests/server.js`)