The decimal point '.' is changed to locale's decimal point
before/after JSON conversion to make C standard library's
locale-specific string conversion functions work correctly.
All the tests now call setlocale(LC_ALL, "") on startup to use the
locale set in the environment.
Fixes GH-32.
json_int_t is typedef'd to long long if it's supported, or long
otherwise. There's also some supporting things, like the
JSON_INTEGER_FORMAT macro that expands to the printf() conversion
specifier that corresponds to json_int_t's actual type.
This is a backwards incompatible change.
