[iOS] Slide to view when SelectedIndex or SelectedItem changed.

[rdaniel0]

I'm not sure if this is a bug or feature request.

I downloaded the example project.
When a tab button is clicked in the tabs view, the page is changed immediately instead of sliding to it.

I expected it to slide the view in, like in your demo images, when I click one of the tab buttons.

[rdaniel0]

I tested Pull request #27 and it works fine in my project.
Please merge and then this can be closed.