#8 Add method to interface not an error

[Stephen Colebourne]

Currently clirr reports adding a method to an interface as
an error. There is a situation when it is not an error.

version 1:
interface SubMap extends Map {
}

version 2:
interface SubMap extends Map {
/**
* Some additional javadoc.
*/
Object get(Object key);
}

This is not an error because the newly added method
overrides exactly one in a previous superinterface.