json-cpp Code

- Added Json::ArrayIndex as an unsigned int to forwards.h
- Modified Json::Value to consistently use Json::ArrayIndex.
- Added int/unsigned int constructor overload to Json::Value to avoid ambiguous constructor call.
- Modified jsontestrunner/main.cpp to use Json::valueToString for Value::asInt() conversion to string.
- Modified Json::Reader to only overflow to double when the number is too large (previous code relied on the fact that an int fitted in a double without precision loss).
- Generalized uintToString() helpers and buffer size to automatically adapt to the precision of Json::UInt.
- Added specific conversion logic for UInt to double conversion on Microsoft Visual Studio 6 which only support __int64 to double conversion (unsigned __int64 conversion is not supported)
- Added test for 64 bits parsing/writing. Notes: those will fail when compiled with JSON_NO_INT64 (more dev required to adapt).