#80 Add withChronology to LocalDate

[x-zero]

Class LocalDate is missing copy method withChronology(Chronology). This means that explicit creation using constructor LocalDate(Object, Chronology) must be used to get an 'adjusted' copy.
Presumably, the method would be able to check whether or not the desired chronology is the same as the current chronology, and return an unmodified object if possible, rather than requiring creating duplicate objects.

[Stephen Colebourne]

I'd be happy to accept a GitHub pull request with tests for this.