Generate javadoc for ScalarTypes

Modern versions of default doclet are issuing this warning:

[WARNING] /home/nite/odl/mdsal/binding/mdsal-binding-test-model/target/generated-sources/BindingJavaFileGenerator/org/opendaylight/yang/gen/v1/urn/opendaylight/params/xml/ns/_default/value/test/norev/BigUnsignedInteger.java:28: warning: no comment
[WARNING] public BigUnsignedInteger(Uint64 _value) {
[WARNING] ^

This particular instance comes from this YANG snippet:

    typedef big-unsigned-integer {
        type uint64 {
            range "4500000000..6500000000";
        }
        default "5000000000";
    }

We need to generate a similar Javadoc as we do for interfaces, i.e. expose at least the YANG snippet from which we generated the type. Note there are two basic cases here:

1. typedef, as above, where we should quote the typedef
2. leaf/leaflist, as in leaf foo { type uint64; range 1..2; }, in which case an inner class is generated, where we should quote the leaf/leaf-list

There are also two different cases in class hierarchy: we may or may not have a superclass. If we do, we need to also reference it in javadoc.