Some update for UI7TabBar, UI7PickerView, UI7NavigationBar, UI7SearchBar

I update some code in my project to make

1. UI7TabBar would use tintColor for selected item title, not the selected image only
2. Using UIAppearance backgroundColor if not nil for UI7PickView, as default iOS7 style background is some kind of transparent, which cause lots of problem when project needs to be compatible with iOS 6
3. Using UIAppearance backgroundColor if not nil for UI7NavigationBar
4. Using placeholder if not nil for UI7SearchBar