just like React img tag but some additional props.

• placeholderSrc,loadingSrc,errorSrc,src props match 4 stage of an image.

1. render to dom : placeholderSrc
2. begin load : loadingSrc,if not set,use placeholderSrc
3. img load success : src,
4. img load error : errorSrc

• onError,onLoad event behave like broswer default.
• getElement return the img element that this component rendered.