Enable use of converters on xs:simpleType

In order to enable use of converter classes to convert to/from enums produced by `xjc`, parse `xs:simpleType` elements for the `jrb:converter` attribute.

This is not the cleanest solution, as it requires users to write a converter class for each enum they intend to use, but given that converters must be stateless there doesn't seem to be a better solution (the converter can't rely on reflection, for example, because it doesn't know the correct enum class at runtime).