width and height attribute doesn't work for img

[xiaohanzhang]

I thing there are two issues here:

1. draft-js-utils version is too low. It only support "src" and "alt" for in 0.2.0.
2. ImageSpan doesn't handle width and height type properly.
   For example: <img width="100" height="100"/> will resolve into const imageStyle = {width: '100', height: '100'} in ImageSpan, and it will be ignored by react.
   And <img width="100px" height="100px"/> will resolve into const imageStyle = {backgroundSize: '100pxpx 100pxpx'}, which is also invalid.