better logic for detecting presence of spree_wombat. No more rescue..end hackery :)

Conflicts:
	spec/lib/serializers/spree/wombat/assembly_shipment_serializer_spec.rb
update readme for usage with spree_wombat
make sure we add spree_wombat to the test group in the Gemfile
added usage with spree_wombat in the readme
count the correct amount of parts in the line_items.
Added model_class def to parts_controller
Fixes #96
Fixes #97
Merge branch '2-2-stable' of https://github.com/spree-contrib/spree-product-assembly into 2-2-stable-lav

Conflicts:
	Gemfile