qml: Added support for attributes in extra

In quakeml Pickler._extra, Added a check for a "_type" key in each value of "extra". Adds the value to the element's attrib dictionary if "_type" is "attribute".

Didn't see a test for 'extra' yet, but this:
```python
e = Event()
e.extra = {'dataid': {'_namespace': ['catalog', 'http://anss.org/xmlns/catalog/0.1'],
    '_type': 'attribute',
    'value': '00999999'}}
c= Catalog(events=[e])
print Pickler().dumps(c)
```
gets you:
```
<?xml version='1.0' encoding='utf-8'?>
<q:quakeml xmlns:q="http://quakeml.org/xmlns/quakeml/1.2" xmlns:catalog="http://anss.org/xmlns/catalog/0.1" xmlns="http://quakeml.org/xmlns/bed/1.2">
  <eventParameters publicID="smi:local/0b8ab6cc-c6de-4e95-bfb3-7fcdbaf08cf9">
    <event publicID="smi:local/e8b8bcf5-629b-4404-b666-8e1728cdc5ba" catalog:dataid="00999999"/>
  </eventParameters>
</q:quakeml>
```