ie8 bug fix

[maggiehe]

IE8 not works well as there are some bugs in image stage,

1. ImageStage.prototype does not have an init() method, but init() was called in attach() method.
2. options should have an imgsrc property, the callback(applySizeConstraints method) need this proterty to calculate xscale, yscale, etc:)

[wangzh1010]

ie8的时候我也遇到了这个问题。
给AbstractStage.prototype加一个init()的空方法。