Serialize DateTimes in ISO8061 format in JSON responses

It seems that setting *any* (non-null) date format on the object mapper causes DateTimes to be serialized in ISO8061 format, i.e.

    static {
        mapper.setDateFormat(new SimpleDateFormat(""));
    }

would cause DateTime objects to be serialized in ISO8061 format too. This appears to be a bug in Jackson version 1.9.10.

I set the format to "yyyy-MM-dd'T'HH:mm:ssz" to try to make it clearer why a date format is being set on the ObjectMapper.
Add a constructor to Job which set enqueuedAt to current time

This constructor uses DateTimeUtils.getCurrentTimeMillis() to create the DateTime object for the enqueuedAt field. This is because it is possible to control the result of this method, and so makes testing the enqueuedAt time of a Job easier in situations where it is set at runtime.
Add tests for enqueuedAt attribute