add_items helper not available on AccompanyingCanvas

[markpbaggett]

While working on a solution for recipe_0014, I noticed that the add_item() helper is not available on AccompanyingCanvas. If you wanted to use the AccompanyingCanvas class and not just Canvas, it would be useful to have add_item() available here.

Expected behaviour:

add_item() is available to AccompanyingCanvas

Observed behaviour:

An exception is thrown when add_item() is used here.

Potential bug location(s):

iiif-prezi3/iiif_prezi3/helpers/add_item.py

Line 2 in aeed7b6

from ..skeleton import (AnnotationPage, Canvas, Collection, Manifest, Range,

[glenrobson]

Done in #200