Schema changes to support image annotation

Hi Jamie,

here's a pull request which made Annotator/Annotorious integration work on my end. There's one caveat, however: the contents of the ``geometry`` field will change according to the shape type. I.e. while it will look like this for rectangles...

     shapes: [{
        type: 'rect',
        geometry: { x: 0, y: 0, width:100, height: 100 }
     }]

...it will hold an array of points:

     [{ x: 0, y: 0 }, { x: 10, y: 10 } , { x: 100, y: 20 } , { x: 0, y: 0 }]

if the shape type is a polygon. I.e. in the mid-term run, it would be cool if there was some schema-free way to store geometries somehow.