Remove old code, create new typedefs
Remove extra brackets
Implement Width, Height and Insets attributes
Implement grouped attributes and keepAllInsets proxy attribute
Avoid setting priority if it is the same

I got runtime exceptions when the priority was changed from 1000 to
1000. This is likely a bug in the NSLayoutConstraint class, but this
workaround is safe.
Don't store `allInsets` attribute as an associated object
Simplify and generalize KeepAttribute class cluster
Fix incorrect relation for Minimum
Add centering attributes
Implement offset attributes, with working example
Fix grouped attributes
Attributes now have readable description with optional `name`
Change parametrized attribute accessors to use blocks
Add NSArray convenience methods
Alignment attributes
Add methods for easy animation of constraints
KeepAssert macro, that can be disabled at any time
Documentation in header of UIView+KeepLayout
Refactoring of attribute accessors
Refactoring of KeepAttribute class cluster