Support polymorphism on schema generation

(moved from: https://github.com/FasterXML/jackson-databind/issues/41 )

---

t would be nice to support polymorphism on schema generation.
Currently, the generated schema doesn't provides information on subtypes with the above case .

```
@JsonTypeInfo(
use = JsonTypeInfo.Id.NAME,
include = JsonTypeInfo.As.PROPERTY,
property = "type")
@JsonSubTypes({
@Type(value = subtypeA.class, name = "A"),
@Type(value = subtypeB.class, name = "B"),
@Type(value = subtypeC.class, name = "C")
})
public abstract class ParentType { }
```