Multiple Categories for K2 1.3.7

# [Bug Fixed] When saving a subcategory the items removed from the #__k2_multiple_categories [Many thanks to Шульга Евгений]
# [Bug Fixed] Class 'Factory' not found after trying to migrate from the "Additional Categories for K2
Joomla! K2 Plugin"
+ Watchful support.
# Minor fixes and improvements.

# [Bug Fixed] The total calculation of K2 items is not correct where these displayed on category list. This affects only the k2 subcategories.
# [Bug Fixed] Call to a member function isSite() on null [Thank you, Gerardo Marín]
# Minor fixes and improvements.