The IgnoreOverflowDigits=true parser always succeeds and because of that it
expects from the digits parser that a number passed as inout argument will be
changed only on success.
Changed negative_accumulator and X3 for consistency.
It will allow to test `extract_int` properly.
Replacing `boost::integer_traits` with `std::numeric_limits` will not hurt
performance because even MSVC 9/GCC 4.1/Clang 3.0 folds `min`/`max` calls
to a constant at compile time and since C++11 they are even constexpr.
