Bootstrap3 : DateTextField - start/end date should use the date time in ISO8601 format

When using DateTextField with a custom format, withFormat("yyyy-mm-dd") for example, then using withStartDate and withEndDate do not work properly.

In the code withStartDate and withEndDate allways use the same Formatter :
```java
    public DateTextFieldConfig withStartDate(final DateTime value) {
        put(StartDate, value.toString(FORMATTER));
        return this;
    }
```

Instead start/end date should use the date time in ISO8601 format (yyyy-MM-ddTHH:mm:ss.SSSZZ).