Use .alert-heading instead of hardcoding <h4>

rename .arrow => .popover-arrow

New rule added in JSCS v1.9.0
Our JS already complies with this.

Remove deprecated tooltip placements

Automate breakpoints

Breakpoints and container widths are now defined as maps.

Also support Ruby Sass

* Separate configs for libsass and sass.
* Sass compiler selected based on `process.env.TWBS_SASS`.
* Travis:
  * Use Gemfile to manage ruby dependencies.
  * Run core tests with both Sass compilers.
  * Only install/cache ruby gems required by the test subset.
* Grunt: `update-gemfile-lock` task a la `update-shrinkwrap`.

Conflicts:
	docs/components/helpers.md
	scss/_utilities.scss

use :disabled pseudo-class instead of [disabled] attribute

Re-add Firefox placeholder opacity workaround

Remove .hidden and .show classes