Add support to exclude serializing empty Strings

Jackson and Gson both support excluding empty Strings when serializing

Jackson Example:
mapper.setSerializationInclusion(Include.NON_NULL)
or
@JsonInclude(Include.NON_NULL)

I cannot seem to find a way of doing this with LoganSquare...

It would be nice to set this on a class level (via annotation) and/or by the LoganSquare.JSON_FACTORY (so ALL @JsonObject classes will be included)