WIP: df rendering using templates + conditional formatting for HTML  
closes #3190.

Provides conditional formatting and CSS styling of HTML tables for dataframes using code.
The plan is to clean up and package the functionality behind a nice API in some pandas namespace,
perhaps with a dotted API, and ship in 0.14.

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.
- Related https://github.com/pydata/pandas/pull/5374. need to have format-specific escaping function as part of the process.
- Equivalent functionality to formatters
- Should be able to "freeze" constructed views and use them as functions of other df
  (Example: highlight 3 largest values in "price" column for each frame in a panel).
- Use to define default style for html output (bootstrap-like for visual integration with ipnb).
