Implement layout margins relative arrangement.

[cabeca]

layout margins relative arrangement

This PR implements the layout margins relative arrangement feature. It defines two new properties:

@property(nonatomic) UIEdgeInsets layoutMargins;
@property(nonatomic, getter=isLayoutMarginsRelativeArrangement) BOOL layoutMarginsRelativeArrangement;

The first one just defines the missing layoutMargins property on iOS7.
The second one switches the layout relative to margins.

I also added some buttons to visualize the feature in the example App.

bonus

• In the process of developing this feature I also solved a small bug with the center alignment: because it didn't define any constraints relative to the edges of the stackView, the content views could easily overflow the stackView boundaries. This was fixed.
• I also added a background view to help visualize the margins. Now that the stackView's backing layer is a CATransformLayer we could not set a backgroundColor on it to help visualize the layout. I fixed this by placing a background view behind the stackView and constraining it to have the same size and center of the stackView.

[harlanhaskins]

I'm using this fork now, and it's very nice and works as intended. 👍

[nsomar]

This looks great!!, Sorry for the later reply, Can you add a couple of test cases to verify the margin alignment? Otherwise I will add them my self in the weekend.

[cabeca]

Yes, I’ll try to add them in the weekend.

[harlanhaskins]

Any updates? I'm anxious to try this fork with the latest updates to master 😁

[nsomar]

Hey, Me too :D.
I had little time this weekend but I definitely want to merge this asap.
I will try to add some specs tomorrow in case @cabeca cannot add them.

[cabeca]

Sorry about the delay. I added the missing tests to the project.

[nsomar]

Great, this addition is super cool!
I will merge this branch by hand since I want to fix the failing test on iOS 7

[nsomar]

Merged! Thanks!