public BaseSliderView image(String url){
        if(mFile != null || mRes != 0){
            throw new IllegalStateException("Call multi image function," +
                    "you only have permission to call it once");
        }
        mUrl = url;
        return this;
    }
set a url as a image that preparing to load
@param url
@return
library/src/main/java/com/daimajia/slider/library/SliderTypes/BaseSliderView.java