Method.getReturnType does not return fully qualified name

org.jboss.forge.parser.java.Method.getReturnType returns a String, rather than a JavaClass, because "there's no way to tell if the type we are returning is actually something we can get the sources of" (Lincoln).

I think this is fine, but if we're using Strings it needs to be the fully qualified name, or else there's no way to understand what it actually is. At the moment it just returns "Long" instead of "java.lang.Long" and "Date" instead of "java.util.Date" etc.