Fixed nested tab elements li activation

Added closest li instead of direct parent so that you can nest multiple elements within tabs, such as button groups
Add nested kbd element styles for actual keys or inputs
Dropdown menu in justified button-group

It seems that dropdown-menu in justified button-group appears at the far
left of the group. Check the problem in [Bootstrap
site](http://getbootstrap.com/components/#btn-groups-justified).
fires closed.bs.alert *after* DOM detach()

fires closed.bs.alert *after* element is removed from DOM.
Previously it fired while the element was still attached.
Fixed navbar radius of example at docs
Changed last-child to first-child
As noted by @jerem the first child will actually be the last shown
Update alert.js

Added call to remove() after event fires to clean up data as per discussion at #12379
bump grunt-css-flip to ~0.2.2
Merge pull request #13433 from watilde/fixed-navbar-radius-of-example-at-docs

Fixed navbar radius of example at docs
Improve accessibility of close buttons in modals & alerts

Adds textual caption to the close buttons. The "X" multiplication sign isn't very meaningful to screenreaders.

Credit: https://github.com/paypal/bootstrap-accessibility-plugin
Per:
    https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#alert
    https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#modal-dialog
document #13390
mark Android Firefox as officially supported
Wall of browser bugs: formatting tweaks
Merge pull request #13495 from twbs/android-firefox

Officially support Android Firefox
Merge pull request #13493 from twbs/close-btn-access

Improve accessibility of close buttons in modals & alerts
include RTL CSS CDN URL in Download docs
add RTL CSS files to "What's included" section of docs
emphasize that bootstrap-rtl.css & bootstrap.css should not be used together
replace js scrollspy test for default behavior
Fix .visible-print deprecation version #.