Handle nameReference as well as type-based element defs

This is a fantastic resource!

By the way, another fun corner case (beyond extensions and ids) is the way that elements within FHIR can "borrow" there datatypes from other elements. This is how recursion is implemented for example, in [Composition.section.section](http://hl7.org/implement/standards/fhir/composition-definitions.html#Composition.section.section)

```
     <path value="Composition.section.section"/>
      <definition>
        <short value="Nested Section"/>
        <formal value="A nested sub-section within this section."/>
        <min value="0"/>
        <max value="*"/>
        <nameReference value="Composition.section"/>
```

Your fhir-elements.xml currently does:
```
  <element path="Composition.section">
      <min value="0"/>
      <max value="*"/>
      <type value=""/>
   </element>
```


E.g. from an earlier effort in this direction, I was generating structure-describing XML [like this](https://github.com/jmandel/fhir-rdf/blob/master/generic/xslt_defs.xml#L23077).

I've submitted a quick fix here.