Resolved issue #1760 -  getThumbnail() now returns an object instead of a path.

I have added the class Asset_Image_Thumbnail which is returned when getThumbnail() is used. This resolves issue #1760.

The new class allows for easy access to the width, height and other properties of the generated thumbnail, which is often needed.

Additionally there is a very useful getHTML() that will output a <img>-tag with all the right attributes for easy embedding in HTML templates.

Backwards compatibility is present, the __toString() method returns the path as before. The method getThumbnailConfig() is still present but should be deprecated in favor of getThumbnail()->getConfig().