Set properties instead of hard coded defaults

[grgcombs]

Once I get settled in a little more with a project I'm working on, I'll start issuing pull requests here. But in the interim, I'll post some things that I'm altering and other requests.

Right now you've got hard defaults for leftPanningOffset and leftViewControllerWidth, etc. I'm creating properties for each of these that can be set at runtime.

I'm also writing in a compatibility layer to mimic the method names and functions of the UINavigationController, like popToRootViewController and initWithRootViewController. This makes it easier to pose in hybrid apps.