Add method to control the leading and trailing spaces when auto-distributing views.

Currently when using this method in NSArray (AutoLayout):

```Objective-C
 - (NSArray *)autoDistributeViewsAlongAxis:(ALAxis)axis withFixedSpacing:(CGFloat)spacing alignment:(NSLayoutFormatOptions)alignment;
```

The spacing value not only specifies the spacing between the views but also a leading and trailing spacing that was unexpected when I began using it. Instead of altering this behavior, I've added a more specific method that allows you to control the leading and trailing spacing around the first and last view respectively. 

The new method I've added is:
```Objective-C
- (NSArray *)autoDistributeViewsAlongAxis:(ALAxis)axis withFixedSpacing:(CGFloat)spacing withLeadingSpacing:(CGFloat)leadingSpacing withTrailingSpacing:(CGFloat)trailingSpacing alignment:(NSLayoutFormatOptions)alignment;
```