aggregate by time
I wrote a function to aggregate data by time, for instance to get average profiles over a day or week if you have a yearly timeseries. 
It's nothing highly complicated, most of the functionality is already in pandas.   But as I could not figure out how to do it with the current API in a single line, I thought this function may be useful to add.

The function and some code to demonstrate it can be found here:
https://www.dropbox.com/s/wffzg11djtrdz23/aggregate_ts.py

Sorry I couldn't do this in the more professional way with a pull request.  I  hope this helps.

(Wes, you'll remember we discussed this very briefly on Euroscipy)
