[objc] Enable class name customization via RegisterAttribute

[zgramana]

As a library creator, I want to enable fine grained control over the public API that I present so that my users find my library intuitive and easy to use. One aspect of this is conforming to naming conventions in the objc/Cocoa community. Xamarin.iOS's RegisterAttribute provides a natural vehicle for this, objcgen just needs to honor it.

[chamons]

I duplicate filed this here:

#612

A rough impl draft was here: #366

[chamons]

#483 is a duplicate as well

with a different patch:

#482