Adding support for custom Jackson module

Provide a mean to register Jackson module through XML configuration. By default the JsonConverter is configured as it was with the overridden class MappingJackson2JsonHttpMessageConverter but through XML. The idea was to give a simple mean to register custom Jackson modules. I use the same principle as the jpaProperties property : 
```
<bean id="jacksonModules" parent="resthubJacksonModules">
      <property name="sourceList">
            <array merge="true" >
                 <value type="java.lang.Class">org.resthub.web.module.CustomModule</value>
            </array>
      </property>
 </bean>
```

Thus, it keeps the PageResponse module and add the customs ones.