Fixed membership types with monthly duration - support rollover day

Add support for configuring a Fixed Period Rollover Day for membership types with Monthly duration.

For example, membership type with 12 month membership duration. I want signups on or after the 20th of the month to get the current month 'free'. Set that Fixed Period Rollover Day to 20. Now membership signups on or after the 20th get start date of current month, and end date of beginning of next month + 12.

Discussion here: http://forum.civicrm.org/index.php/topic,19358.0.html

— Original Issue ----
According to the documentation (wiki.civicrm.org/confluence/display/CRMDOC41/Membership+Types):
"If you want to ensure that memberships last 12 full months by rounding up a partial month (for people who join in the middle of the month), you should select 13 month duration and fixed period. A user who joins on November 15, 2009 will have a start date of November 1, 2009 and an end date of November 30 2010."

The above is true for a new membership. Renewing this membership while it is still current, however, extends the end date to December 31, 2011 (13 months), which effectively adds an extra month of membership.

The "13 month fixed" duration choice seems to be a special case intended to provide a year-long membership that starts on the 1st of the month and ends on the last day of the same month of the following year. Membership renewal should recognize this case and simply extend the end date by 12 months. That is, you should get the partial month when you join, but not when you renew (because the end date has already been adjusted to provide the full month).

See discussion at: forum.civicrm.org/index.php?topic=19358.msg80534

Patch against trunk version attached.