Change CSS classes name for floating images to match those generated by reST figure and image directives

Markdown has no directive for captions so lets use what reST has
to offer.

Markdown users will have to add raw HTML anyway, to have captions with
their images. These classes can be used in it.

static/css/elegant.css

@@ -629,26 +629,26 @@ ul.list-all-articles li time {
     font-style: italic;
 }
 /* Use following classes to add floating images to your post */
-div .float-image {
+div.figure {
     margin: 1.5em;
     text-align: center;
 }
-div .float-image img {
+div.figure img {
     width: 100%;
 }
-div .float-image p {
+div.figure p.caption {
     color: gray;
     font-size: .6em;
     margin: 0;
 }
-div .float-image p a {
+div.figure p.caption a {
     border: none;
     color: gray;
 }
-div .float-right {
+div.figure.align-right {
     float: right;
 }
-div .float-left {
+div.figure.align-left {
     float: left;
 }