No longer any need to handle days before the first day of the year in
month_from_yday; it doesn't work consistently anyway.
Fixed inconsistent handling of the leap day in Year.place: When using
the force flag and going from February 24th on a leap year to a
non-leap year it backed up to the 23rd, while Month.place keeps the
day. The behavior of Month.place seems slightly more natural.
Cleaned up the leap day handling in Month.place a bit. Functionality
is the same.
Fixed propagation of the force flag to the inherited place functions
in both Year.place and Month.place.