Many features, improvements and fixes

New features:

• Add serialize field to @BuiltValueField. Use this to tag fields to skip
  when serializing.
• Add wireName field to @BuiltValue and @BuiltValueField. Use this to
  override the wire name for classes and fields when serializing.
• Add @BuiltValueEnum and @BuiltValueEnumConst annotations for specifying
  settings for enums. Add wireName field to these to override the wire names
  in enums when serializing.
• Add support for polymorphism to StandardJsonPlugin. It will now specify
  type names as needed via a discriminator field, which by defualt is
  called $. This can be changed in the StandardJsonPlugin constructor.
• Add BuiltListAsyncDeserializer. It provides a way to deserialize large
  responses without blocking, provided the top level serialized type is
  BuiltList.
• Add built in serializer for Uri.

Improvements:

• Allow declaration of multiple Serializers in the same file.
• Explicitly disallow private fields; fail with an error during generation if
  one is found.
• Improve error message for field without type.

Fixes:

• Fix generated builder when fields hold function types.
• Fix checks and generated builder when manually maintained builder has
  generics.
• Fix name of classes generated from a private class.
• Fix builder and serializer generation when importing with a prefix.