Add method sizeToFit()

[lucdion]

Add a method sizeToFit().
The method adjust the view's size based on the result of the method UIView.sizeToFit()

Example:

view.pin.center().sizeToFit()

[antoinelamy]

It would be nice to remove sizeToFit() in the SizeCalculable protocol, I made a mistake in putting it there. According to the documentation, sizeToFit() only calls sizeThatFits with the current view bounds and assign that size (probably with some validations).

[antoinelamy]

Or we can also leave it as is but I feel that there is a bit of repetition there since no view should override sizeToFit() directly but do so for sizeThatFits() instead.

[lucdion]

@antoinelamy I will keep it that way in this PR, but we have a discussion about this.

[cajurabi]

@lucdion @antoinelamy my 2cents would be to remove from the SizeCalculable protocol for UIKit consistency. Thanks for the great work on PinLayout and FlexLayout.