email-stats: originally authored by John Wise (30 Nov 2016)

Fetches your mail through IMAP and calculates some time statistics.
By default, it will cache your dates in the file, headers.cpkl.
Assumes that your IMAP server uses SSL with port 993.

1. Complete daily history with 7-day and 30-day running averages.
2. Emails/day/hour versus hour in day
3. Emails/day versus day of the week
4. Emails/day versus date in month
5. Emails/day versus week of the year
6. Punchcard analysis: showing emails versus day and hour