Require Spree 2.1.0.beta
Remove references to methods removed in spree/spree

So that we can actually boot the app
Spree::Variant no longer has a `on_hand` attribute

Instead stock it should be check through variant stock items. This makes
it possible to go through checkout without raising errors
Allow test to run

Update factory girls and test helpers path
Remove specs that give no value to the extension

They were basically testing spree core functionalty. No need to maintain
those here
Hook parts into new Spree stock API

So that they are included on shipments and have their inventory units
changed as expected. Most of the previous code was completely removed
since it didn't make sense anymore
Allow both Spree 2.1 and 2.0 to use master branch

No need to create another branch now if we can keep compatibility with
both Spree versions on the same branch
Improve test coverage with features spec
Display included parts on completed order page
Update README
Fix backend shipments UI

Use deface to override parts of the shipment partial as it considered
every item in the manifest to be an order line item. But that's might
not be the case once the store starts building bundled products

Fixes #31
Use an icon for the Parts tab
Add failing spec for orders in backend:

This adds a failing spec for orders with both:
- A product assembly
- An item that is a product that also belongs to that assembly
Viewing such orders in the backend causes an sql exception due
to the ambiguos semantics of the assemblies_for method in such
contexts.
Fix variant.assemblies_for query
Check for parts stock items instead if product is assembly
Advise users the extension is still beta for Spree 2.0
Display product bundles on backend orders UI
Remove before_payment code from CheckoutController

Otherwise it removes the product bundle items on the order because
they're not included in the shipment (unshippable)

There's a PR for this on spree/spree but I'm not sure it
will be accepted, see https://github.com/spree/spree/pull/3193
Fix rails command

Migrations should be generated in the extension dir, not in the dummy
app
Group inventory units by line item on shipment manifest

Update Package::ContentItem to also have a line item.