Use inventory_unit.line_item to fetch shipment line items

Resolve issue where a shipment item_cost would be 0 because it could not
find any line item associated with the shipment

Fixes #50
Fixing layout in product admin interface

Fixes #51
Support 2.1.0.beta
Remove 2-0-stable compatibility from master in README
Add newline to install_generator.rb

Fix issue where installing subsequent gems causes syntax error in app/assets/javascripts/admin/all.js
Allow bumping selenium-webdriver
Fix variant eager loading in Shipment `manifest`

See https://github.com/spree/spree/pull/3687
Allow users to dont split packages

Replay changes from https://github.com/spree/spree/pull/3618
Fix order_id assignment in OrderInventory

See https://github.com/spree/spree/pull/3712
Replace :confirm option by data: { confirm: "text" }

:confirm will be deprecated as of rails 4.1
Replace deprecated LIKE constant for arel matches
fix syntax error
Depend on Spree ~> 2.1
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