Add a few more checks for the @Bean annotated methods.

[shochdoerfer]

Add a few more checks for the @bean annotated methods, e.g. the methods need to be publicly accessible, otherwise the methods cannot be called from the AnnotationBeanFactory. The generator should throw some exceptions when an "error" in the configuration was found.

[shochdoerfer]

As with #10 Disco is able to deal with protected methods. This might affect the "error detection".

[shochdoerfer]

See also #69 for some more ideas, e.g. checking generated method names for duplicates.