Add pandasnotebook to misc-pandas

This PR updates the excellent misc-pandas lesson by Michael Hansen with an ipython notebook translation by Michael Hansen. 

The history is from https://github.com/fishjord/swc_hhmi

The changes are
* restoration of the weather-timeseries datafile
* translation to ipynb
* addition of exercises
* answers to exercises in separate file

There remains duplication of content between the html rendering of the earlier copy of the lesson and the ipynb translation, but these changes restore a better version of the lesson.

Notebook viewer:
http://nbviewer.ipython.org/urls/raw.github.com/wltrimbl/bc/add-pandasnotebook/lessons/misc-pandas/an-introduction-to-pandas.ipynb