Add configurable position of tab bar

I'm working on an interface that wants to use a tab bar positioned at the [top of the view controller's view](https://f.cloud.github.com/assets/14554/755546/3f86f66e-e5b5-11e2-82e2-d26d87f61ee5.gif). I thought I'd open a pull request that added a new initializer, `-[AKTabBarController initWithTabBarHeight:position:`], and see if this was compatible with the general direction of the project.

There's a [bit of awkwardness](https://f.cloud.github.com/assets/14554/755562/8dc67002-e5b5-11e2-852d-9427f0925347.gif) with hiding the tab bar within a `UINavigationController`. I haven't worked on a solution because even if someone has a need for a top-aligned tab bar, he/she probably shouldn't use it in conjunction with a nav bar :smiley:.