NEW: CPLayoutConstraint shoudBeArchived

Currently forced to YES in nib2cib. Defaults to NO (cocoa doc) in code.
Minimize calls to the worker/solver when size changes. Also added CPLayoutConstraint -setConstant: it just creates a new constraint with a new constant in the solver and removes the previous one
Fixed: NSLayoutConstraint nib2cib support in xcode 6.1
NEW: CPControl &CPCheckbox intrinsicContentSize

CPCheckbox needs a _minimumFrameSize or a sizeThatFits
cleanup
New: CPLayoutConstraint -setPriority:

All properties are now immutable except constant and priority. per
cocoa.
Better -description. -description affected by constant/priority changes.
Bindings to constant, priority, description are possible.
See IBGenerated… Test.
Fix Tests