add normals param and change skip_rate to set Percentage

Just add general params for normal and change the skip_rate way.
![normals1](https://cloud.githubusercontent.com/assets/3803922/3036372/14040cf8-e0a9-11e3-8ef0-c88d01735969.png)
![normals2](https://cloud.githubusercontent.com/assets/3803922/3036385/35ce8bd8-e0a9-11e3-9563-007a13d17bbe.png)
![normals3](https://cloud.githubusercontent.com/assets/3803922/3036374/1404786e-e0a9-11e3-8373-ecef3b83a102.png)
![normals4](https://cloud.githubusercontent.com/assets/3803922/3036375/140699fa-e0a9-11e3-87ef-283bc8e2c121.png)