Pull request: GSoC'13 Wiselib NS-3 Port -- Position and distance facet

Dear all,

This is the pull request for position and distance facets in the Wiselib NS-3 port project. Features in those two facets include:

1) position facet: get and set position for a single node. Here, setting position is an additional support which is not defined in position facet concept.

2) distance facet: calculate the 3D distance between two nodes.

I also create a wiki page to show how to use those two facets: https://github.com/dizhizhou/wiselib/wiki/Wiselib-NS3-port

If you have any comments on the codes and wiki, please feel free to contact me. 

Best regards
Dizhi