implemented special characters L,W,#

[mariusbancila]

The README has always listed L, W and # as supported; the parser never understood any of them. This implements all three.

0 30 23 ? * 5#2    the second Friday of the month
0 30 23 ? * 5L     the last Friday of the month
0 30 23 L * ?      the last day of the month
0 30 23 LW * ?     the last weekday of the month
0 30 23 15W * ?    the weekday nearest the 15th

W never crosses into another month, and L alone in the day-of-week field means Saturday, both as in Quartz. Each qualifier applies to a single value, so 5#2,6#3 is rejected. They remain invalid in the other fields.

cronexpr now carries two option structs beside the bitsets, since the day these select depends on the month; the existing day-by-day scan consults them.

Testing L also uncovered an unrelated bug: 0 30 23 L 2 ? skipped 2012 entirely, because setting the month while the day was still 31 rolled February over into March. Fixed here too, same family as #30.

New test_special.cpp covers all three, with dates worked out from the calendar rather than read back from croncpp.

Fixes #18 and #14. Also delivers what #36 asked for, which was closed earlier by rejecting these characters instead of implementing them.