jbock 5.8

In order to avoid certain staleness problems with gradle's "isolating" annotation processing model, some features had to be removed.

• Remove the Converter annotation.
• The converter attribute must now point to a static member class of the command class (see note).
• Forbid inheritance of abstract methods.

Note: Effectively, it is still possible to define a "standalone" converter class, by writing a small static inner class which implements Supplier<StringConverter<X>> and returns an instance of the "real" converter class from its get method. For example, see how BigIntegerConverter is used in CustomConverterCommand.