Backward compatibility for python <=2.6

[satyam-m]

-bash-4.0# ./psig -p 8451
Traceback (most recent call last):
File "./psig", line 229, in
main(args)
File "./psig", line 177, in main
print_signal_info(args, args.pid, max_pid_length)
File "./psig", line 157, in print_signal_info
for k,v in obtain_signal_info(pid).iteritems():
File "./psig", line 120, in obtain_signal_info
signal_info = collections.OrderedDict()
AttributeError: 'module' object has no attribute 'OrderedDict'