Refactorings in Preparation to Framework Support

Before everything else get merged, I would like to get these changes in.

Those reduce ambiguties in preparation to framework support by getting rid of the attribute name `library` instead of that attributes are now named consequently on base of their class names:
* `PBXNativeTarget` →  `native_target`
* `Target` →  `target`

These change don't incorporate the Hooks-API so far, so it shouldn't break backwards-compatibility.
imho, these specs should take `@gen.generate` and strip on newlines, chomp, select lines of length `> 0` and compare as an array on imports. What do you think, @mrackwitz ?