Add DescribedImageExtractor to get Images on Described simplification

When simplifying the Images block on Describeds delegate extraction of
the Set of Images used to DescribedImageExtractor.
Merge branch 'image-attributes' into stage

* image-attributes:
  Add DescribedImageExtractor to get Images on Described simplification

Conflicts:
	src/main/java/org/atlasapi/output/simple/DescribedImageExtractor.java