Fix issues with the test script so it works under distcheck too.

Add tests and descriptions for the functions in json_util.c

To better distinguish between entry->k and entry->v being const within linkhash, but non-const outside, add lh_entry_v() and lh_entry_k() accessors.
Make lh_entry->k const.

Change %d to %llu and add cast to unsigned long long for size_t arguments,
otherwise compilation will fail with errors like:

test1.c:70:15: error: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Werror=format=]

%zu is avoided to stay compatible with old libc versions (like old Visual
Studio).

The user might want to use the userdata for something different, so the
serializer should ignore it by default.

Explicitly setting the serializer to json_object_double_to_json_string will
still make it interpret the userdata as a format string.

Add utility function for comparing json_objects

Tests added.

Do some basic checks on ints, doubles, strings, arrays and "complex" objects.

Tightening the number parsing algorithm

Some badly formated "numbers" could get partly parsed,
resulting in truncated results instead of raising an
error.

Examples :
 '1.2.3'      -> (double)1.2
 '2015-01-15' -> (int)2015

This patch is not perfect (ex: input can still end with a 'E', which
is forbidden by json.org doc) but should avoid non-sensically
formated input.

Tests added.

Add json_object_free_userdata() and json_object_userdata_to_json_string() too.

Conflicts:
	json_util.c

Adjust test_null to check for this case.
See also http://bugs.debian.org/687269

This is particularly useful when replacing values in a loop, since it allows
 the key used by json_object_object_foreach to continue to be used.