Add command for generating conversation stats.
Implement conversation stats by date.
Add test for conversation date stats.
Group conversations by month, not day, and format the date using XLS friendly date stamps.
Python 2.6 doesn't have writeheader.
Implement message_counts_by_date.
Rename by-date stats to by-month.
Rename variables to reflect that we are gathering data by month.
Update sub-command help to explain that stats are being collected by month.
Add option for setting date format.
Add test for date format and switch to using call_command in tests.
Factor out stats writing and date formatting.
Merge branch 'feature/issue-1065-stats' into develop