Add pg to Gemfile
Fix build in postgres

Wrong api and different sql ordering were causing failures in postgres
Move `part?` to Variant model

The assemblies association defined in Product was not correct. We don't
associate products with products, we only product with their parts which
are variants

Not completely sure but the `part?` method in Variant might help to
either prevent multi-level nesting or provide that feature right, as
mentioned in #45

fixes #45
Display line_item price in line_item_manifest, not variant price

Use line_item instance method to show price

Remove useless file
Instantiate IU states as empty hash when iterating grouped line_items
Add test case for iu states issue
Fetch units by line item in OrderInventory `verify`

Related to https://github.com/spree/spree-product-assembly/issues/58

Once product assembly is installed Spree should always take into account
line_items when searching for inventory units for, well, a given line
item
Dont depent on db ordering in spec

It was causing random failures probably due to database ordering

Conflicts:
	spec/models/spree/line_item_spec.rb
Consider variant and line item when removing inventory

When removing inventory units we need to filter them per line item as
well. Some orders might have the same product sold as a part and within
a bundle

Fixes #58
Depend on Spree 2.2.0.beta
Update stock_contents to no longer rely on shipment.adjustment
Update order_inventory_assembly.rb

when adding a new line item to a complete order, the changed_attribute value for quantity returns nil. So we parse it as an integer so nil will become i and fix nil issues.
Remove overrides no longer needed

Spree core was modified to improve assemblies support, see
https://github.com/spree/spree/pull/3991
More update to reflect recent change in spree core api
Remove MOAR overrides
No need to decorate OrderInventory anymore

As of https://github.com/spree/spree/commit/e5492937823d7470831a973c108e674c44f69a64
OrderInventoryAssembly now inherits from OrderInventory

Let us remove a whole MOAR override \o/
Dont require spree/frontend in gemspec
Move assets to spree/spree#4050 friendly position
Remove ShippingRate.backend scope reference from _stock_contents partial

Fixes #64