code: String getElementKeyString(Element element) {
  		JsonKey key = element.getAnnotation(JsonKey.class);
  		JsonModel model = element.getEnclosingElement().getAnnotation(JsonModel.class);
  		if (!"".equals(key.value())) {
  			return key.value();
  		} else if ("".equals(key.value()) && key.decamelize()) {
  			return decamelize(element.toString());
  		} else if ("".equals(key.value()) && model.decamelize()) {
  			return decamelize(element.toString());
  		} else {
  			return element.toString();
  		}
  	}
docs: Get JSON key string.
  @param element
  @return JSON key string
  @author vvakame