pandas materials (time series!)

At @gvwilson's suggestion, I've started writing up some materials for a pandas workshop.

This is just a sketch for now, but the idea with this first part is to show off pandas' time series capabilities. In just a few lines of code, you can read some time series weather data into a dataframe and find the average temperature each month.

Here's the [notebook on nbviewer](http://nbviewer.ipython.org/github/jvns/bc/blob/pandas-tutorial/python/pandas/01-weather.ipynb).

Thoughts?