use RubyDateFormat to format Date, yeah!
add formatting letters in javadoc to RubyDateFormat formats
add missing format %g (short weekyear) to RubyDateFormat
fix RubyDateFormat %c, blank padded-day, not zero-padded
fix RubyDateFormat FORMAT_CENTURY: use getYear() so it does not depend on Chronology

* In ISOChronology, it is fine, but in GJChronology, it's century + 1.
* Still a problem for the year right now with GJChronology (but no test!)
RubyDateFormat: handle ignored POSIX modifiers as MRI
handle extra date formats in RubyDateFormat