Implement Accounting 52/53 week calendar

Squash commits as referenced in PR #24 

Implementation of the 52/53 week calendar used for accounting/retail/manufacturing purposes, such as detailed in IRS Publication 538 and the International Financial Reporting Standards.
 All tests passing.
 ...I'd actually appreciate some critique on names chosen for things. In particular I'm a little worried about my use of "Accounting", although that is the most likely use for this calendar (retail and manufacturing tend to use it in that capacity as well as other areas).
 Because this calendar requires setup (as no two companies are the same),  AccountingChronology.getCalendarType()  returns  null  (no LDML, unlikely to have an entry).  AccountingChronology.getId()  returns just  "Accounting" , which only differentiates it from other calendars, not other accounting calendar instances ( toString()  does output the entire object state, and  equals() / hashCode()  are overridden to work properly).
 Obviously I can't set this up for  ServiceLoader  (there is no default implementation, thank you). Should I provide an explicit parser, or just rely on regular serialization?
Use IllegalArgumentException rather than DateTimeException for NPE checks (multiple occurrences)