Add support of long and unsigned long types for ValueObject

[elsid]

This is useful for OpenMW to store std::int64_t and std::uint64_t user values inside osg::Object. C++ doesn't guarantee long to be 64 bit wide but it's common to have std::int64_t as an alias for long and std::uint64_t for unsigned long. It's possible to store such values as std::string, but has overhead and produces additional code with copying int to string and back.