[Serusers] nathelper updated

[Maxim Sobolev]

Folks,

I've updated nathelper module in trunk. The following changes were made:

- Applied patch from Bogdan-Andrei IANCU <iancu at fokus.fraunhofer.de>, 
which allows to use nathelper without usrloc module if natpinger isn't used;

- fixed natpinger for clients without explicit transport specified in 
the Contact header field, which means that transport is UDP;

- fix_nated_contact() now ignores non-UDP contacts.

Please let me know if you will see any unusual problems with the module.

-Maxim