Introducing image filtering

The Okfn plugin has an addImage() function which builds an okfn-image-plugin around the image.
Now, before doing this, we're checking certain properties of the image, because sometimes it is unecessary to try to use annotorious in an image:
- If the image element has the css property -[webkit|moz|ms]-user-select set to none. This means that the image is not selectable,  we shouldn't try to make selections on it either.
- The AnnotoriousImagePlugin can be configured (in the options parameter) to have minWidth and/or minHeight property, which is meant to filter out too small images which are not suited for image annotations.