Image Annotation Extractor

This pull request adds an ImageAnnotationExtractor that extracts image annotations stored using the ImageAnnotator system on the Commons (https://commons.wikimedia.org/wiki/Help:Gadget-ImageAnnotator). 
I'd argue that we're saying that "http://commons.wikimedia.org/ImagePath#xywh=1,2,3,4" is a Thing (an Annotation) on "http://commons.dbpedia.org/resource/ImagePath" -- this could be used for any kind of annotations, including those without an explicit area (such as image metadata, say). "hasAnnotatedArea" suggests to me that an annotated area exists, but it's not immediately obvious what the datatype should be. We could potentially use hasAnnotatedArea to describe the area annotated (_image_ dbo:hasAnnotatedArea "x=10,y=10,w=100,h=100") -- would that help?

Another possibility is http://dublincore.org/documents/dcmi-terms/#terms-hasPart instead of hasAnnotation, but I like how hasAnnotation fits in perfectly with the already-defined dbo:Annotation class.