Add image info and free ratio

Hi,

I've added:

* image information, fixes [https://github.com/fengyuanchen/cropper/issues/6]
* Ability to have free ratio, fixes [https://github.com/fengyuanchen/cropper/issues/5]

Plz. merge :+1: 

Note: To have free aspect ratio, set:

    aspectRatio: false

In cropper options