added possibility to add pan gestures to left and right panel

Two new properties:
```
recognizesLeftViewPanGesture
recognizesLeftViewPanGesture
```
If set to ```YES```, it will allow the user to close the left or right
panel by pan gestures on the corresponding view. Can be useful if a panel is filling the entire screen so that the centerpanel cannot be "dragged" or closed by tapping the menu button.