collisions prevention for pois

Implements #216 

![poi3](https://cloud.githubusercontent.com/assets/1421652/3099244/ade4f6a8-e5f8-11e3-9a75-5adfc2874859.jpg)
![poi](https://cloud.githubusercontent.com/assets/1421652/3099242/addfcf20-e5f8-11e3-9a2b-006490ab33e8.jpg)
![poi2](https://cloud.githubusercontent.com/assets/1421652/3099243/ade0f22e-e5f8-11e3-8a4a-4f88bfa731c9.jpg)