[Fix #461] oneOf options must inherit union class common part

[fjtirado]

Fix #461
This slightly change the generated code public API.
Properties in POJOs that contains both properties and oneOf are moved to a parent class which is inherited by the oneOf alternative classes. This is done that way because reading two nested classes from the same json node is pretty tricky and will move the complexity to the generated serializer/deserializer (reinventing Jackson essentially)

Just one example, previously we have

public class Schema implements Serializable, OneOfValueProvider
{

    /**
     * SchemaFormat
     * <p>
     * The schema's format. Defaults to 'json'. The (optional) version of the format can be set using `{format}:{version}`.
     * 
     */
    @JsonProperty("format")
    @JsonPropertyDescription("The schema's format. Defaults to 'json'. The (optional) version of the format can be set using `{format}:{version}`.")
    private String format = "json";
    private final static long serialVersionUID = 7823919531911950336L;
    private Object value;
    private SchemaInline schemaInline;
    private SchemaExternal schemaExternal;

after this PR, we will have

@JsonSerialize(using = SchemaUnionSerializer.class)
@JsonDeserialize(using = SchemaUnionDeserializer.class)
public class SchemaUnion
    implements OneOfValueProvider<Schema>
{

    private Schema value;
    private SchemaInline schemaInline;
    private SchemaExternal schemaExternal;

and

public class Schema implements Serializable
{

    /**
     * SchemaFormat
     * <p>
     * The schema's format. Defaults to 'json'. The (optional) version of the format can be set using `{format}:{version}`.
     * 
     */
    @JsonProperty("format")
    @JsonPropertyDescription("The schema's format. Defaults to 'json'. The (optional) version of the format can be set using `{format}:{version}`.")
    private String format = "json";
    private final static long serialVersionUID = 7823919531911950336L;