Latest ipython master complains of exceptions in df._repr_html_

ipython/ipython#4792

df.repr_html currently raises ValueError. if IPython agree to the
proposed behaviour, we'll change it to NotImplementedError to restore things to normal.

ipython/ipython#4832 resolves this and allows the error to be supressed
by users of older pandas using the standard mechanism in the python warnings module:

import IPython
warnings.filterwarnings('ignore',category=IPython.core.formatters.FormatterWarning)