Pull request of GSoC'13 NS-3 port projects: Radio facet

Dear all, 

This is the pull request of radio facet for NS-3 port project in GSoC'13. It only includes the code in the Wiselib part. The code in NS-3 part can be found at https://github.com/dizhizhou/ns-3-dev-git. A wiki page is added for the details of radio facet (https://github.com/dizhizhou/wiselib/wiki/Wiselib-NS3-port).  

If you have any comments on the code, please do not hesitate to contact me.

Regards
Dizhi