Fixed usage of json-c lib >= 0.10.

json-c changed the handling of 64 bit wide integers and got rid of
json_type_int64 which we used. This broke compilation on systems
with current json-c libraries.
Updated pkg-json.c to work with all released versions (I hope.)
Merge pull request #4 from zesstra/json64-fix

Fixed usage of json-c lib >= 0.10.