added support for inheritance

This is a solution to the limitation that otto only processes methods of the immediate type, disregarding any annotated super methods. This topic has been discussed [here](https://github.com/square/otto/issues/26).

This pull request adds an annotation to the public API of otto which allows the user to explicitly declare classes for which otto should check super classes for annotations (producers/subscribers).

Credits for the original implementation of this pull request go to @deadfalkon.