Added a writeDatesAsTimestamps property to the JodaMapper

Since the WRITE_DATES_AS_TIMESTAMPS SerializationFeature is highly related to the JodaMapper, I added a property on JodaMapper to set this configuration.

This also has the benefit of making Spring configuration much simpler.

    <bean id="jsonObjectMapper" class="com.fasterxml.jackson.datatype.joda.JodaMapper">
        <property name="writeDateKeysAsTimestamps">
            <value type="java.lang.Boolean">false</value>
        </property>
    </bean>