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.

Extend the run_output_test() function so we actually can pass command line
 parameters and so we can support different output files for the same test
 executable.
Also provide some hints about what to do if a test fails (i.e. set VERBOSE=1).