Add support for generation sources into a framework.

- Add a protoc objc option (generate_for_named_framework) to set the name of
  the framework all generated sources will be in.
- Tweak some comments/naming to make it clear what is the Protobuf framework
  vs. the framework for generated code.
- Update the objc README to document the new generation option to protoc.

This is working towards https://github.com/google/protobuf/issues/1457.