ENH: proof of concept notebook for conditional formatting of dataframes HTML export

closes #3190.

Provides conditional formatting and CSS styling of HTML tables for dataframes using code.
The plan is to package the functionality behind a nice API in some pandas namespace,
perhaps with a dotted API.

Unlike my first attempt from 6 months ago which turned into a rabbit hole, this is a much simpler approach
that came together in a few hours and supports everything I care about.

See the ipython notebook in the root directory, or on [nbviewer](http://nbviewer.ipython.org/github/y-p/pandas/blob/PR_html_conditional_formatting/pandas_HTML_styling.ipynb).

Feedback welcome.