Pull request: GSoC'13 Wiselib NS-3 Port -- Clock facet

Dear all,

This is the pull request for the clock facet that I implement in NS-3 for Wiselib. It only includes the code in the Wiselib part. The code in NS-3 part can be found at [1].  A wiki page is added for the details of such clock facet [2].

If you have any comments on the codes and wiki page, please feel free to contact me.

[1] NS-3 part: https://github.com/dizhizhou/ns-3-dev-git
[2] wiki page: https://github.com/dizhizhou/wiselib/wiki/Wiselib-NS3-port