Screen edge pan

Adds two properties:  `paneDragRequiresScreenEdgePan` and `screenEdgePanCancelsConflictingGestures`, evolved from #48. See below for documentation.

```objective-c
/**
 Whether the only pans that can open the drawer should be those that originate from the screen's edges.
 
 If set to `YES`, pans that originate elsewhere are ignored and have no effect on the drawer. This property is designed to mimic the behavior of the `UIScreenEdgePanGestureRecognizer` as applied to the `MSDynamicsDrawerViewController` interaction paradigm. Setting this property to `YES` yields a similar behavior to that of screen edge pans within a `UINavigationController` in iOS7+. Defaults to `NO`.
 
 @see screenEdgePanCancelsConflictingGestures
 */
@property (nonatomic, assign) BOOL paneDragRequiresScreenEdgePan;

/**
 Whether gestures that start at the edge of the screen should be cancelled under the assumption that the user is dragging the pane view to reveal a drawer underneath.
 
  This behavior only applies to edges that have a corresponding drawer view controller set in the same direction as the edge that the gesture originated in. The primary use of this property is the case of having a `UIScrollView` within the view of the active pane view controller. When the drawers are closed and the user starts a pan-like gesture at the edge of the screen, all other conflicting gesture recognizers will be required to fail, yielding to the internal `UIPanGestureRecognizer` in the `MSDynamicsDrawerViewController` instance. Effectually, this property makes it easier for the user to open the drawers. Defaults to `YES`.
 
 @see paneDragRequiresScreenEdgePan
 */
@property (nonatomic, assign) BOOL screenEdgePanCancelsConflictingGestures;
```