Change syntax for identifying identity columns

Change the way identity columns are identified at the <class> mapping so that we (in futuram) could deal with field names that contain spaces.

The new syntax could look similar to

<class name="ctf.jdo.tc9x.Product">
<identity>
<field name="id1" />
<field name="id2" />
</identity>
/<class>

making go away with the identity attribute on the <class> element.