[TIMOB-9186] iOS: Implement backgroundImage for toolbars in iOS using setBackgroundImage:forToolbarPosition:barMetrics:

This is a suggested improvement for toolbars. iOS 5 now has ability to apply a proper BG image to toolbars. This means we can now make nice toolbars without the Glassy effect. Applied fix both to Ti.UI.iOS.Toolbar as well as to Window.setToolbar. Why no pull request? Because I'm not 100% confident in what I've done. It works for me, and is a good starting point - but someone with a more obj-c/titanium experience should probably review it. For example, you'll see some duplication. Feedback welcome! I've attached a diff of the changes I made (against the 2.0.X branch) as well as example app.js showing usage.