Added total-downloads.py example

It's just a simple example of chart to present total number of downloads: installers or packages.

Before launching this script there is a necessity to clone appropriate source of data (this could be available only for internal developers of Continuum Analytics).
>  If df.date is really a datetime then call .date() instead

I couldn't find the appropriate way to handle this. I will rewrite the code according to this suggestion. Thanks.