The style of the tab bar. Supported values are 'default', 'black'. Use 'black' instead of setting barTintColor to black. This produces a tab bar with the native iOS style with higher translucency.
Specifies tab bar item positioning. Available values are:
fill - distributes items across the entire width of the tab bar
center - centers item in the available tab bar space
auto (default) - distributes items dynamically according to the user interface idiom. In a horizontally compact environment (e.g. iPhone 5) this value defaults to fill, in a horizontally regular one (e.g. iPad) it defaults to center.