Add `#group_by_month_of_year` grouping

Allows for grouping by months (Jan to Dec) regardless of year.
Add month_of_year to README
Added day_start option to month_of_year
Merge branch 'zorab47-add-month-of-year-grouping'
Add day_of_month grouping

Closes #40
Closes #45
Merge branch 'rmm5t-day_of_month'
Updated readme and changelog
Do not quote column name
Version bump to 2.3.0
Updated readme
Lock down dependencies correctly
Fixed tests
Added series: false option for arrays and hashes - for #63