Added methods to store constraints for later use, like in animations

I often find myself creating constraint properties so I figured it would be easier to add a way to store these by name right on the view:

```objc
NSLayoutConstraint *backgroundViewTopConstraint = [[backgroundView pinToSuperviewEdges:JRTViewPinBottomEdge inset:-110] firstObject];
[backgroundView storeConstraint:backgroundViewTopConstraint forKey:@"backgroundViewTopConstraint"];

...

[UIView animateWithDuration:0.35 delay:1 usingSpringWithDamping:0.8 initialSpringVelocity:0.2 options:UIViewAnimationOptionCurveEaseInOut animations:^{
    NSLayoutConstraint *backgroundViewTopConstraint = [self.backgroundView constraintForKey:@"backgroundViewTopConstraint"];
    backgroundViewTopConstraint.constant = 0;
    [self.backgroundView.superview layoutIfNeeded];
} completion:nil];
```