Enable or disable swipes from left and right

Added two new properties following project conventions.

```objc
@property (nonatomic, assign) BOOL allowLeftSwipe;  // defaults to YES
@property (nonatomic, assign) BOOL allowRightSwipe; // defaults to YES
```

In <code>-gestureRecognizerShouldBegin:</code> I added a check for the state of each property and the direction of the gesture to determine if the gesture should begin.

```objc
// determine if right swipe is allowed
if (translate.x < 0 && ! self.allowRightSwipe) {
    return NO;
}
// determine if left swipe is allowed
if (translate.x > 0 && ! self.allowLeftSwipe) {
    return NO;
}
```

This is a pretty simple change that adds some functionality that I needed in my application. My usage was to have the right VC only be visible with selection of a button press, not a gesture. Other uses could be for disabling/enabling swipes during runtime.