Used property getter for shouldDrag instead of ivar for more flexibility

Instead of using the ivar ```_shouldDrag```, I propose to use the property accessor instead.
This way, user can specify in his/her implementation easily if a cell can be swiped or not. 

User can just override the getter for shouldDrag in his custom MCSwipeTableViewCell implementation, then return a value based on the current state, e.g. return NO if table view is in edit mode (```self.isEditing``` is YES). User does not need to manually set each cell's ```_shouldDrag``` value when edit mode has been set to YES (no need to loop through all cells then set new ```_shouldDrag``` value).

Any thoughts?