Negative integer values are not handled properly

[carneyweb]

If I have some json that looks something like:

{
"foo" : -1
}

The value of -1 gets saved in the compilable output as the 2's complement unsigned value.

The problem is at line 76 in json2cpp.cpp:

return fmt::format("std::int64_t{{{}}}", value.get<std::**uint64_t**>());

Change that to std::int64_t and everything works great.