Fix styling of superscripts and subscripts. Closes #1924.

IPython/frontend/html/notebook/static/css/boilerplate.css

@@ -34,6 +34,9 b' time, mark, audio, video {'
   vertical-align: baseline;
 }
 
+sup { vertical-align: super; }
+sub { vertical-align: sub; }
+
 article, aside, details, figcaption, figure,
 footer, header, hgroup, menu, nav, section {
   display: block;