Have gourmet/test/test_ica_se_plugin.py also test for cooktime parsing.
Factor schema.org parsing out of ica_se_plugin.py into a newly created schema_org_parser.py.
Have ica_se tests check for presence of category field.
Remove obsolete gtk import from gourmet/importers/generic_recipe_parser.py.
Scrap more schema.org/Recipe fields.
Make ica.se importer aware of a recipe's yields property.
Fix allrecipes.com importer, by extending our newly created schema_org_parser.

Also add a test gourmet/test/test_allrecipes_com_plugin.py which is capable of
testing the importer offline, using gourmet/test/recipe_files/allrecipes_com.html.
(Trying to integrate such an offline test into the existing, commented-out
allrecipes.com test in gourmet/test/testImporters.py proved a nightmare as
the importer code is tightly interwoven with the web page download and interactive
importer functions.)

Fixes #261