Added support for active state images, fixed tab widths and UISplitViewController support

Changes
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Active Images
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Added a property to add an active image name
Modified the AKTab class to support this. A little rough, but clearly commented.

Fixed width tabs
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Added support to for fixed width tabs, which are automatically centred.

UISplitViewController
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Added support for setting the tab content from the master controller (view controller:0) of a UISplitViewController.