Add a hudRadius property, support custom the HUD view's corner radius.

Easy and very useful!!
![2013-07-01 6 53 56](https://f.cloud.github.com/assets/1063805/729745/a047b000-e23c-11e2-8be5-8df800c9ea5f.png)
![2013-07-01 6 53 31](https://f.cloud.github.com/assets/1063805/729746/a4d0c08a-e23c-11e2-90d6-65c20689a8d0.png)