Natbib (and presumably biblatex) bibliography commands create their own section. Since these are in frame environments, we have an incompatibility with the `\AtBeginSection` macro which creates a special frame when a new section occurs. (We can't have a frame inside another frame.) This change disables `\AtBeginSection` inside bibliography slides. Thinks to Yihui Xie for bringing the problem to my attention. This supersedes #145. See discussion there.