Update jpdl XSD with fork transition conditions

The jpdl xsd needs to be updated to reflect the fact that transitions in fork activities can have nested condition elements. Looking at the code for ForkActivity, it's apparent that <fork> activities do evaluate conditions for outgoing transitions.

Example:

<fork name="fork1">
<transition to="always_do_this"/>
<transition to="sometimes_do_this" >
<condition expr="#

"/>
</transition>
</fork>