P2P NAT类型检测
NAT检测工具:
http://sourceforge.net/projects/stun/files/WinStun/0.96/WinStunSetup-0-96.msi/download?_test=goal
检测结果说明：
Independent Mapping, Independent Filter = Fullcone NAT
Independent Mapping, Address Dependent Filter = Restricted Cone NAT
Independent Mapping, Port Dependent Filter = Port-Restricted Cone NAT
Dependent Mapping = Symmetric NAT
可选STUN服务器：
stun.qq.com
stun.1und1.de
