Add config to place tab bar at top of AKTabBarController
Note positioned initializer w/in README
Add screenshot of iPhone portrait w/ top tab bar
Include top bar bar screenshot in README
Add commented-out example to toggle tab bar position

Someone checking out the repo can comment and uncomment the lines to see
the different initializers' effects.
Added a fixed height setting for the tabbar, as discussed in #40
Added support for individual tab background images.
Merge branch 'top-tab-bar' of https://github.com/andrewsardone/AKTabBarController
AkTabBar will change it's autoResizingMask according to it's position now.
Added a sample how to display an view in the center of the tabbar (#43).