Added more options for sliding operation

Wanted to add a few more options to the way the center view controller and side menus slide past one another. Here's a breakdown of what's different:

* Depreciated `menuSlideAnimationEnabled` and `menuSlideAnimationFactor` properties. They still work, but there will be some unclear results from mixing new and old properties, so they're marked as depreciated and will throw a warning if used.

* Created new internal method `setControllerOffset`. An offset of 0 means closed, negative would open the right menu, and positive would open the left menu. All three views are positioned from this view so that there is no need for methods aligning views to one another.

* Created `showMenuOverContent` property that allows the menus to slide in front of the center view controller. The default is still to slide behind the center.

* Created `menuParallaxFactor` property to control the amount of sliding of the side menus. A factor of 0 would leave the menu stationary (default behavior), a factor of 1 would slide the menu directly with the center (like a UIPageViewController), and a factor between 0 and 1 would slide the menu at a different speed from the center, giving it a parallax scrolling effect. This is only utilized when menus are behind the center view.

* Created `contentParallaxFactor` property to control the amount of sliding of the center view. The values go from 0 to 1 and work as described for the `menuParallaxFactor`. This is only utilized when menus are in front of the center view.

* Created separate shadows for all three views. Shadows only show up on the view in front. When the menus are in front, their shadows gradually fade in and out when open/closed so there is no shadow when the menus aren't up.

* Remove the locking of directions (resolves #109). When closed, user can start panning right, and still pan left without having to begin a new gesture.

I realize that this is a pretty major change and that there is a lot of code that is being modified. Take a look through it and let me know what you think.

(Go Badgers!)