* Deletes the accordion.less file
* Extends the panel to be collapsible with our JS plugin
* Supports panel color variations

fixes typo introduced in 2b6ec389

Properly hides popover titles in IE8 because apparently it doesn't
accept the `:empty` selector. /cc @fat @cvrebert

A rebase (against soon-to-be 3.0.0-rc.1) & squash of
https://github.com/twbs/bootstrap/pull/7786 AKA
https://github.com/twitter/bootstrap/pull/7786
originally by
@seriema
@mokkabonna
@jojohess

Rebased by @cvrebert