Major rework of 64 integer support: 64 bits integer are only returned when explicitly request via Json::Value::asInt64(), unlike previous implementation where Json::Value::asInt() returned a 64 bits integer. 

This eases porting portable code and does not break compatibility with the previous release.

Json::Value::asLargestInt() has also be added to ease writing portable code independent of 64 bits integer support. It is typically used to implement writers.