DataFrame.to_csv/read_csv inconsistency
This is not a bug; maybe it is more of a philosophical issue.

When I download data from Yahoo, mimicking the code on p. 139 of the book (python for data analysis) it creates a data frame.  The index for this data frame is a time series index - timestamps provided by Yahoo.  

If I save this DataFrame to disk using to_csv and then read it back using read_csv the resulting data frame now has a 'normal' range type index [0,1,2...n] and a new column labelled 'Date' which now contains the dates.

This means that code written to analyze this data needs to be different depending on whether it has been saved or not.  I can deal with it as is, but I think this is not the best design; DataFrames should be invariant under saving.

Just my $ 0.02.

Respectfully, 

Greg St. George
