#22 Support @Convert on @Attribute members

[Daniel Demus]

It seems you can only specify a converter on a member decorated with @Element. Seeing as a converter takes an InputNode, the restriction seems arbitrary. Making sure the converter receives a compatible InputNode and can produce compatible output is the responsibility of the Converter implementation.

[Daniel Demus]

Also, unless there is some trickery going on, this should only require putting something like

if (element == null)
{
    if (type.getAnnotation(Attribute.class) == null)
    {
        throw new ConvertException("Element annotation required for %s", type);
    }
}

somewhere in the private Convert getConvert(Type type) method in ConverterScanner.java