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).
`newdf = df.date.apply(lambda x: '%d-%02d-%02d' % (x.year, x.month, x.day))`? But what's the point if you `strptime` it later on. pandas should work fine (?) with dates. If `df.date` is really a `datetime` then call `.date()` instead.