fix  a NSString type-defect

While I try the Demo on **Lion 10.7.4** and **Xcode 4.6.3 (4H1503)** , the **copy** declared of the tooltip property in `BFIconTabBar.h`  cause the following  problem:

![8c3b5678-b20c-4118-ae40-674c652939a9](https://f.cloud.github.com/assets/2641747/2427439/a2d79148-abe8-11e3-8fe4-4144d54ac9f3.png)