Fix GJChronology.plus/minus across cutover and year zero

Fixes #28
When subtracting a number of years from a date in the GJChronology there are two considerations
The cutover date might be crossed, and year zero might be crossed (there is no year zero in GJ)
Previously, each were handled separately, but not together. Now it is fixed
As part of this change, the minimum cutover instant was set to 0001-01-01 (Gregorian)