Support using NSKernAttributeName for auto-kerning with UILabel

[ZevEisenberg]

From the iOS 8.3 UILabel documentation:

To turn on auto-kerning in the label, set NSKernAttributeName of the string to [NSNull null].

To research

• What exactly does "auto-kerning" mean in this context?
• Does it have performance implications?
• Does it work only for UILabel? What about UITextView, UITextField, Core Animation, and Core Graphics?

[ZevEisenberg]

Turns out NSKernAttributeName works like this for UIKit:

• Non-zero NSNumber: adjusts tracking
• @0: disables font kerning (ugly, only do this if you have a very good reason)
• [NSNull null] - enables pretty font Kerning. It's not clear from the documentation, but this is necessary only for manual attributed string drawing. In UIKit, even if you don’t set this property, this is the default behavior.

To consider for BonMot: should we always use [NSNull null], just in case the string is being used in Quartz drawing? Or should we provide a separate accessor, -attributedStringForBitmapDrawing or similar?