New free public NatPunchthrough server

[Luke1410]

The free RakNet NatPunchthrough server was shut down in 2015. SLikeSoft provides a replacement for that one now which is accessible at natpunch.slikesoft.com (free of charge).

While designed to work with SLikeNet (https://www.slikenet.com/), it's fully backwards compatible with RakNet 4.081/4.082 and can be used by existing projects.
To help with integrating the new NatPunchthrough service into your project, we created a pull request to ease the transistion #135 .

Please note that the service is currently in alpha stage. See also the announcement in our forum: https://www.slikesoft.com/forum/viewtopic.php?f=4&t=164857