Center .icon-prev and .icon-next

Center .icon-prev and .icon-next the same way .glyphicon icon are.
And this is the html markup:
<a class="left carousel-control" href="#carousel-main-menu" role="button" data-slide="prev">
<span class="icon-prev" aria-hidden="true"></span>
</a>
Example: minor image compression improvements.

[ci skip]
Add .media-object in docs
Merge pull request #15258 from Quy/media-object

Add .media-object in docs
Merge pull request #15244 from zalog/patch-2

Center .icon-prev and .icon-next in carousel
clarify collapse example with link and button triggers; also mention classes
Merge branch 'master' of github.com:twbs/bootstrap
nuke the other incomplete example per @ianw
Move inline CSS to docs.css.
Carousel example: move the Base64 images outside of the HTML.

Use a simple blank.gif and change the background color instead of duplicating stuff.