refactor recurrence iterator
* separate expansion and filtering
* filter during expansion
ensure we don't build the error message on every iteration
Merge branch 'master' into refactor_filters
few minor improvements
fix typo in a comment
remove WeekDay.toCalendarDay
few minor optimizations
fix javadoc 
remove unsed import
add method to set the year of an instance
improve instance tests
code style
some minor improvements
added javadoc
added fast paths fpr certain typed of rules (needs evaluation)
minor optimizations
javadoc fixes
fix java doc
fix javadoc
fix FreqIterator
check for infinite loops
add another check to some BYSETPOS tests
improve rule parser slightly
fix BYMONTHDAY expansion for yearly rules
update & fix tests
switch to storing 0-based day of week in instances instead of values as
used by Calendar