Json::Value constructor: Accept any arithmetic type.

Allow construction of Json::Value from size_t, short, etc.
Fixes issue #64
Mostly out of curiosity: why this `is_specialized` here? According to [cppreference](http://en.cppreference.com/w/cpp/types/numeric_limits/is_integer), `is_integer` is false for non-specialized types. Maybe that's not guaranteed for future versions of the standard?