Added descriptor API

[ennerf]

Added descriptors for #55. New public API:

class SomeGeneratedMessage extends ProtoMessage {
  public static Descriptor getDescriptor();
} 

interface Descriptor {
  FileDescriptor getFile();
  String getName();
  String getFullName();
  byte[] toProtoBytes();
}

interface FileDescriptor {
  String getName();
  String getFullName();
  String getPackage();
  byte[] toProtoBytes();
  List<FileDescriptor> getDependencies();
  List<Descriptor> getAllContainedTypes(); // everything that is in this file
  List<Descriptor> getAllKnownTypes(); // all types from this file and all dependencies
}

The PR also removes the recently added SomeGeneratedMessage::getDescriptorProtoBytes. The feature was experimental and didn't work as intended.

[PeterJohnson]

Works great for me!