Add support for xs:choice

Implemented this to the extent possible in the 1.0 architecture. The big problem is that there's no real way to support a true choice for marshalling, since the components don't provide any way for the container to check if they're present.

What about support for marshalling choice, when choice elements are mapped into the same field?:

class ...{
private Vehicle vehicle;
}

<mapping ...>
<structure field="vehicle" type="Vehicle" map-as="Car" usage="optional"/>
<structure field="vehicle" type="Vehicle" map-as="Truck" usage="optional"/>
.
.
.
</mapping>

<...>
<car id="1"/>
</...>

or

<...>
<truck weight="1"/>
</...>

This case seams to be similar to the heterogenous collections...

The above (Maros's comment) can already be done in several ways, such as by defining an abstract mapping for the Vehicle class which is extended by a concrete mapping each Vehicle subclass. There's then no need to list these as explicit alternatives for the "vehicle" field - they'll automatically be handled based on the actual type of vehicle found at runtime (or the actual element name, when unmarshalling).