Allow different background image

I added a background property and the user can now set their own background (defaults to the ```darkPattern.png``` file).

I also added a few public initializers to throw the ```UIImage``` down the chain from the ```HHTabListController``` to the ```HHTabListTabsView```.

Everything is still backwards compatible. You can init with 
```obj-c
[[HHTabListController alloc] initWithViewControllers:x]
``` 
but now you can initialize with 
```obj-c
[[HHTabListController alloc] initWithViewControllers:x backgroundImage:[UIImage imageNamed:@"whatever.png"]]
```

Let me know what you think! It's been a while since I've written Objective-c, so the code may be a little rusty.